[[caption-width-right:304:[[Webcomic/ScandinaviaAndTheWorld America's Hat]] no more.\\
[[http://www.deviantart.com/art/Canada-Invades-40094493 Image]] by [[http://groggie.deviantart.com/Groggie Groggie]]]]

->''"Heard about UsefulNotes/{{Houston}}?''\\
''Heard about UsefulNotes/{{Detroit}}?''\\
''Heard about UsefulNotes/{{Pittsburgh}}, PA?"''\\
-- '''Music/TalkingHeads''', "Life During Wartime"

A plot device that depicts the United States as a target of foreign occupation by another (nation-)state or states. The work will usually focus on the heroic efforts of either or both the [[UsefulNotes/YanksWithTanks US Armed Forces]] and the LaResistance as they try to defend their homeland.

The background of the invasion usually varies on when the work is set. If it is set pre-1990, it is usually a Cold War that got hot or some other form of AlternateHistory, such as Nazi Germany and/or UsefulNotes/ImperialJapan carving an isolationist America up once they've finished conquering the rest of the world. TwentyMinutesIntoTheFuture will sometimes depict a (dystopian) USA that is no longer a Super-Power but merely a Great (e.g. as it was in 1900) or Second-Rate (e.g. as in 1850) or Third-rate (e.g. 1812) Power that would genuinely be threatened by an invasion from a rival power. NextSundayAD settings will usually be very similar to the Twenty Minutes setting, usually featuring a [[MakeTheBearAngryAgain Russian]] or [[ChinaTakesOverTheWorld Chinese invasion]]. The US's nuclear arsenal almost never gets employed during these stories, despite [[CriticalResearchFailure the fact it'd be a deterrent as per the whole 'MAD' thing]], though its non-use is often hand-waved away by the proliferation of some [[Main/AppliedPhlebotinum new technology]] and/or [[AWizardDidIt magic]] - or by the governments simply going "better to try reclaiming what's ours than doom the whole world".

If done right, can make for an interesting plot. However, can also be {{Anvilicious}} if it is too blatant a rip-off of any real-life war, especially one in which the US was actually involved.

See also, MexicoCalledTheyWantTexasBack for a more specific example of this trope. DividedStatesOfAmerica sometimes goes hand-in-hand, as other nations come into the country to act as peacekeepers, to secure international interests, or to conquer the splintered US. Goes hand-in-hand with OccupiersOutOfOurCountry, FallenStatesOfAmerica and DayOfTheJackboot. May or may not involve a WashingtonDCInvasion.

SubTrope of DifferentStatesOfAmerica.

'''Note that the trope only counts for ''human'' invasions on US soil, not [[AlienInvasion alien invasions]].''' (And not ''[[IThoughtItMeant that]]'' kind of "[[IllegalAlien alien]] invasion" either.)


* ''Film/RedDawn1984'': A classic example, in which the USSR invades the Western United States and a group of high schoolers' efforts to stop it. [[Film/RedDawn2012 The remake]] has North Korean invaders (originally [[ChinaTakesOverTheWorld Chinese]], but changed in order to have it marketable there).
* Parodied in ''Film/CanadianBacon'': three random schlubs thinks that the US is being invaded by Canada. HilarityEnsues.
* Somewhat obviously, ''Film/InvasionUSA1952'' and to a lesser extent ''Film/InvasionUSA1985''.
* ''Film/ThePhiladelphiaExperimentII'': David Herdeg is sent to an AlternateHistory where the Nazis won UsefulNotes/WorldWarII and occupied the U.S.
* ''Film/SixStringSamurai''. Soviet army remnants show up on the path to Vegas. They aren't in particularly good shape (they ran out of bullets some four decades before the movie takes place), but then no-one else is either.
* In ''Film/EscapeFromLA'', a united, [[DirtyCommunists Shining Path]]-led Latin America is on the verge of invading the US.
* {{Discussed|Trope}} in ''Film/HotTubTimeMachine''. Blaine's gang thinks that the time traveling main characters are actually Soviet spies, due to their odd behavior, and the modern-day gadgets that he found in their bags. He mentions the film ''Film/RedDawn1984'', which he owns a poster of in his room.
* Discussed in ''Film/PearlHarbor'' during Roosevelt's first meeting with his advisers after the namesake attack. One general, in underlining just how poor a state the US military is in, states that if the Japanese invaded at that moment they would get as far east as Chicago.

* ''Literature/TheManInTheHighCastle'': Set after the fact, this book depicts an alternate 1960s in which the US was taken over by Japan and Germany from the West and East coasts respectively. The Rocky Mountain States is a Japanese puppet.
* ''The Fall Of A Nation'' by Thomas Dixon (author of ''The Klansman'', aka ''Literature/TheBirthOfANation''). Written prior to US involvement in UsefulNotes/WorldWarI, it has the United States refusing to intervene in the Great War, which stalls to a stalemate. Peace is declared, but it's actually a ruse to allow the combined armies of Europe to invade America.
* ''Literature/TheMouseThatRoared'': The Duchy of Grand Fenwick declares war on the U.S. with the intent of losing immediately and raking in the reparation money. So they send over a small army in Medieval armor and spears, expecting to surrender upon reaching American soil. [[SpringtimeForHitler It doesn't quite work out that way.]]
* Happens as a NoodleIncident in ''Literature/TheWarAgainstTheChtorr''. A Fourth World "army of economic liberation" lands in the southern United States and promptly gets massacred due to the US secret RobotWar technology. Not to mention they US has rigged the computer chips in the weapons they've been [[BatmanGambit selling internationally for the past few decades to detonate upon command]].
* C.M. Kornbluth's novel ''Literature/NotThisAugust'' begins with the surrender of the U.S. to Soviet and Chinese forces. The novel itself is about the resistance movement.
* Gloriously depicted in Floyd Gibbons' THE RED NAPOLEON, where the US and Canada make a last stand against HordesFromTheEast led by a [[YellowPeril Communist dictator and alleged descendant of Genghis Khan]] who has already conquered the rest of the world -- and the Americans win.
* Robert Conroy's AlternateHistory novel ''1901'' concerns a relatively limited invasion of the Northeast in the title year by the forces of [[UsefulNotes/ImperialGermany Kaiser Wilhelm II]]. The Kaiser's objective here is not to conquer the United States outright, but to force the U.S. to turn over its newly-won colonial possessions of Puerto Rico and the Philippines by essentially holding New York City and surrounding areas for ransom. He also intended to confiscate the U.S. Navy, which America ''wouldn't need'' once it had no overseas territory. Unfortunately (for the Germans), the Kaiser didn't reckon with UsefulNotes/TheodoreRoosevelt...
* Conroy likes to play with the scenario of (usually) German invasions of the USA. His novel ''1920: America's Great War'' posits a scenario in which the U.S., having remained neutral (and the Central Powers thus having won the Great War) is faced with an assault by Germany into California from a German-allied Mexico, and his ebook ''North Reich'' has Hitler deciding to not declare war on the U.S. after Pearl Harbor, instead concentrating on knocking Great Britain out of the war, occupying Canada and invading the United States from the north circa 1943.
* ''Lightning In The Night'', a novel by Fred Allhoff originally serialized in ''Liberty'' magazine during 1940, recounts, in pulp-magazine style, an invasion of the United States by a grand alliance of [[ThoseWackyNazis Nazi Germany]], UsefulNotes/ImperialJapan and [[SovietRussiaUkraineAndSoOn the Soviet Union]] circa 1945.
* Eric L Harry's ''Invasion'' has Chine invading the good old USA thanks to US disarmament. Apparently the blinkered government continued to draw down the US military despite the Chinese conquering the world by ''going West'' (having nuked Israel and beseiged Europe).
* The apocalyptic novel ''Resurrection Day'' is set in a USA where the Cuban Missile Crisis led to a nuclear exchange. With the Soviet Union destroyed, Communism fallen, and the USA crippled, the power vacuum leads to a resurgent Britain reversing its post-imperial decline - blamed on American interference - and plotting to avenge the whole 1776 business by grabbing back its North American colonies. And then some. The "Resurrection" of the title is Britain's rebirth as an undisputed world superpower, if only by default as the only country not touched by nuclear holocaust.
* Rita Mae Brown's novel ''Dolley'' deals with the context for only actual historical invasion of the United States, by the British Army and Royal Marines during the war of 1812.
* US is just one of the [[EarthIsABattlefield many countries]] invaded by [[TheReptilians the Race]] in ''Literature/{{Worldwar}}''. Traveling through the country becomes dangerous, supply lines are cut off, people are forced to make do with what they can scrounge. Some of the states end up in Race hands, but, by the end of the main series, the Race vacate the country as part of a peace settlement with the [[InsistentTerminology not-empires]] that manage to hold them off (e.g. US, USSR, Nazi Germany, Japan, UK). Oh, and some cities are nuked by both sides.
* In ''Literature/AxisOfTime'', Hawaii ends up being taken by the Japanese in the AlternateHistory UsefulNotes/WorldWarII thanks to the use of 21st century tech. The islands are eventually recovered by the US, [[spoiler:but the tyrannical Japanese governor slaughters 90% of the population out of spite]].

[[folder:Live-Action TV]]
* ''Series/StarTrekEnterprise'': Several episodes featured an alternate 1940s in which the Nazis, equipped with alien technology, were able to capture portions of the East Coast.
* ''Series/{{Sliders}}'':
** The second episode featured a Soviet-occupied USA.
** Another episode showed a [[MexicoCalledTheyWantTexasBack Mexican-occupied California]], after the US lost the Mexican-American War.
** One episode features a world where the US lost the Revolution and never existed, so the country is all part of England.
** Yet another episode briefly features a French-controlled USA. In this reality, Napoleon won over in Europe, never sold Louisiana to the US, and eventually conquered the rest of America.
* The miniseries ''Series/{{Amerika}}'' shows a Soviet-occupied US ten years after the invasion. Subverted though in that the US surrendered without much of a fight after an EMP pulse took out most communications. Most Americans are more interested in getting on with life than resisting. ''Amerika'' was a work that was developed as an apology to [[UsefulNotes/PoliticalIdeologies conservatives]], particularly the [[UsefulNotes/RonaldReagan Reagan Administration]], ''Film/TheDayAfter''. The latter showed how terrible a nuclear war would be for the (many) survivors; the former argued how a Soviet victory would also be unacceptable.
* Parodied by ''Series/SaturdayNightLive'' as "[=Amerida=]", a vision of what America would be like after a Canadian invasion and takeover.

* As the storm clouds gathered over Europe and the Far East, PulpMagazine hero ''[[http://en.wikipedia.org/wiki/Operator_No._5 Secret Service Operator #5]]'' (1934 - 1939) fought attempts by various foreign armies from South America, [[ThoseWackyNazis Europe]] and the [[YellowPeril Orient]] to conquer the United States. The events are completely over-the-top as befits the pulp genre, except for the time the Japs destroy an entire city (Philadelphia) with their evil atomic bomb. [[HarsherInHindsight Only Orientals would do such a dastardly deed...]]

[[folder:Table-Top Games]]
* ''The Price of Freedom'' [=RPG=] from West End Games, inspired by the film ''Film/RedDawn1984''. After the Soviets develop an orbital defense system that neutralizes America's strategic nuclear weapons, Soviet occupation forces enter and take over the U.S. The {{PC}}s are American freedom fighters who wage a guerrilla war against the invaders.
* ''[[http://en.wikipedia.org/wiki/Invasion_America_(board_wargame) Invasion America]]'' (1976) board wargame by [=SPI=] (Simulations Publications Inc.). The U.S. is invaded by three international coalitions: the European Socialist Coalition, the South American Union and the Pan Asiatic League. The popularity of this game spawned a companion piece the next year, ''Objective: Moscow'', detailing invasions of the Soviet Union by various coalitions of the Western Allies and China from the 1970's to the end of the 20th Century.
* ''[[http://en.wikipedia.org/wiki/Fortress_America Fortress America]]'' (1986), a MiltonBradley board game. The U.S. is invaded by three international coalitions: the Euro-Socialist Pact, the Central American Federation and the Asian Peoples Alliance. Hey, wait a minute... The box cover for the first edition of this game contains a portrait of none other than UsefulNotes/SaddamHussein (changed for later editions). This edition of the game is now a collector's item. Also, there are some significant differences between ''Invasion: America'' and ''Fortress America''; among other things, the former game covers the whole North American continent from the Arctic to the Panama Canal, done in typical hexagonal-map-and-paper-counter SPI/Avalon Hill, and the MB game covers only the continental United States and is done in the general style of ''TabletopGame/AxisAndAllies''.
* ''TabletopGame/{{Shadowrun}}''. After the U.S. was split up into the DividedStatesOfAmerica, Aztlan invaded both the southern part of California Free State and the Confederated American States.
* The ''TabletopGame/FreedomCity'' setting for ''TabletopGame/MutantsAndMasterminds'' includes the "Erde" setting where the Axis powers won World War II and used a combination of brain-powered war machines, nuclear weapons, and mystic cataclysms to shatter the backbone of the U.S. resistance. The ''Time of Crisis'' adventure involves the heroes helping LaResistance to neutralize the war machines in the process of fighting a cosmic threat.
* ''SS Amerika'' by 3W Games presents an invasion of North America (like ''Invasion: America'', covering the entire continent) by various Axis forces during the UsefulNotes/WorldWarII era. This is, aside from ''Objective: Moscow'', the largest and most detailed tabletop game of this particular subgenre (four full-sized standard wargame maps), with units at the division level including the entire U.S. Army and Marine Corps WWII order of battle, plus British Commonwealth, German, Italian and Japanese units, and even contingents from various Latin American nations which can come in on either the Allied or Axis side depending on various in-game events or scenario preconditions.
* ''TabletopGame/{{Deadlands}}'': In 1877, the British invade from Canada and capture Detroit in retaliation for American military adventurism along the Canadian border.
* ''TabletopGame/AxisAndAllies'' is a World War II simulation. Invading the US is ''possible'' but pretty unlikely (it's usually easier for the Axis to get the two-capitals win condition by taking out Britain and the Soviet Union).
* The World War II board game ''World in Flames'' has a variant where America is invaded by a victorious Germany called, appropriately, ''America in Flames''.

[[folder:Video Games]]
* ''VideoGame/{{Battlefield}}'' series: The ''Armored Fury'' booster pack for ''Battlefield 2'' depicted maps set in Alaska and Pennsylvania as US forces defended their homeland against Chinese and Middle Eastern Collation Forces. ''Bad Company 2'' has several multiplayer maps set in Alaska, and [[spoiler: the ending depicts Russian forces advancing on the northern border of the US.]] Averted in ''Battlefield 3'', as the levels that take place in New York are about stopping a terrorist attack, not an invasion.
* ''VideoGame/CallOfDuty'':
** ''VideoGame/ModernWarfare'': The second game in the series has several levels set in Northern Virgina and D.C. The invaders are not the ''actual'' Soviet Union, since the game takes place long after the end of the Cold War, but the Ultranationalist party which has taken over Russia in the game's timeline is a pretty effective substitute. The third game has them fighting in New York City as well, before finally being pushed off US soil after their defeat there. Appropriately, the first mission set in the invaded U.S. is called [[Film/RedDawn1984 Wolverines!]], and the achievement for beating it on Veteran is Red Dawn.
** ''VideoGame/CallOfDutyBlackOps2'' plays with this-- the US ''is'' invaded...by its own unmanned combat drones, which have been hacked to serve terrorist leader Raul Menendez's bidding. They manage to get boots on the ground as well, in the form of his [[NGOSuperpower mercenary army]]; LA is successfully laid waste to, and they make it as far as New York City and DC before [[spoiler:Menendez self-destructs the drones to leave America defenceless.]]
** This is also part of the plot for ''Videogame/CallOfDutyGhosts'': The bad guys (in this case a league of hostile South American governments) have hijacked an American superweapon system that is turned on the US, devastating the country and leaving it ripe to a conventional invasion.
* ''VideoGame/{{Homefront}}'': The basic plot of the game is the United States under occupation by a resurgent North Korea.
* ''VideoGame/WorldInConflict'': Set during an alternate 1980s, in which the USSR launched an invasion of the US instead of collapsing as in Real Life. The main battles of the war are occurring in Western Europe and the invasion force that lands in Seattle is just there to keep the USA from reinforcing its NATO allies. The backstory also explains that the Soviets previously attempted to invade New York, but the assault was repulsed. No one expected an attack from the other direction.
* ''VideoGame/TurningPointFallOfLiberty'': Set in an alternate 1950s in which the US stayed neutral in World War II and in which Churchill was killed in an accident prior to the war, causing Britain to fall to Germany, which in effect discourages the United States from ever entering the war, solidifying their stance as a neutral country. Eventually, however, the Nazis invade anyways.
* ''VideoGame/DeusEx'': Never actually seen in-game, but background information, along with some side info that you may or may not run into mention the Russian-Mexican War, in which they attempted to [[MexicoCalledTheyWantTexasBack take back territories captured in the Mexican-American War.]] By the time the game starts, southern Texas is under Mexican control.
* ''VideoGame/EndWar'': Battles can take place in the United States.
* ''VideoGame/RiseOfNations'': In the Cold War campaign, besides starting a [[NukeEm nuclear war]] with them, the Soviets can also stage a conventional invasion of the United States.
* ''VideoGame/ShatteredUnion'': The European Union sends peacekeeping forces to the Washington DC area to secure international interests, whereas the Russian Federation invades and annexes Alaska during the [[DividedStatesOfAmerica Second American Civil War]].
* Took place in ''VideoGame/{{Fallout}}'' backstory just prior to the nuclear war, when the Chinese forces invaded Anchorage. It was eventually repulsed. The ''VideoGame/{{Fallout 3}}'' ExpansionPack ''Operation Anchorage'' takes the player character through a VR simulation of the Anchorage Reclamation.
** Descendants from a beached Chinese submarine crew can be found around what is left of San Francisco.
** Ghoulified Chinese infiltrators can be found in a factory in Washington D.C. in ''3''.
** ''Point Lookout'' has a wrecked Chinese spy submarine as part of one of its sidequests.
* ''VideoGame/CommandAndConquerRedAlertSeries'':
** ''VideoGame/CommandAndConquerRedAlert2'' has an ostensibly puppet Soviet government invade the United States with the aid of psychic powers to neutralize the American nuclear arsenal.
** ''VideoGame/CommandAndConquerRedAlert3'' is more global, but both the Imperial and Soviet campaigns feature invasions of the United States at some point. [[spoiler: Going after President Ackerman at Mt. Rushmore in the Allied campaign after he goes rogue may or may not count.]]
* Eventual end result of the Soviets developing the atomic bomb first and dropping it on Berlin in the back story of ''VideoGame/FreedomFighters''.
* The basis of ''Roadwar 2000'' from 1986, but first the invaders released [[ThePlague A Bioengineered Plague]].
* One trailer for ''VideoGame/SteelBattalion: Heavy Armor'' depicts an American invasion of Manhattan in 2080. The opposing force is implied to be related to the United Nations in some way.[[/folder]]

[[folder:Web Original]]
* In the AlternateHistory story ''Literature/TheFalconCannotHear'', the turning point in the Second American CivilWar comes when Japan invades the West Coast (controlled by the democratically socialist Provisional Government, or "Blue", faction). They do this in the belief that this will provide relief towards Japan's nominal ally, the fascist "White" faction, and also attract the support of the remaining warlords of the collapsed military junta (or "Khaki") faction. It all backfires horribly, as ''all'' the American factions are pissed at the violation of their sovereignty; the Whites face massive political instability due to their government's supposed allegiance with the invaders, while all the other factions [[EnemyMine band together]] to both repel the Japanese and defeat the Whites (who they blame for the invasion). [[spoiler: It also inspires Canada to declare itself NeutralNoLonger, and join TheAlliance.]]

[[folder:Real Life]]
!!! Recorded invasions:
* To date, there have been four attempted invasions of the United States:
** The first and more successful series was executed by the Royal Marines of [[UsefulNotes/{{Britain}} the United Kingdom]] during the UsefulNotes/WarOf1812, who torched all of the Government buildings in UsefulNotes/WashingtonDC except for the Patent Office. There were also other raids through Maine, New York state, and New Orleans, all of which were in response to the United States' invasion and attempted annexation of British North America (Canada) and the US Army's razing of Toronto when this didn't work out.
** The second was by [[UsefulNotes/TheAmericanCivilWar the secessionist Confederate States of America]], when Robert E. Lee led his army into Maryland (in 1862, stopped at Antietam) and Pennsylvania (in 1863, stopped at Gettysburg). In the other direction, some Southerners will still claim (either cheekily or half-seriously) that the defeat of the Confederacy was itself an example, and that 'the Northern [liberal/atheist/socialist/socio-economic] invasion of the South' is still ongoing.
** The third and least significant time was during UsefulNotes/TheMexicanRevolution, when Pancho Villa led a raid on the border town of Columbus, New Mexico.
** The fourth and second-least important was by the Imperial Marines of UsefulNotes/ImperialJapan during UsefulNotes/WorldWarII, who seized some of the Aleutian Islands in UsefulNotes/{{Alaska}} as a part of their strategy for defeating the US Navy. Other territories like Guam and the American-run Philippines had been occupied for several months by that point, and since Alaska wasn't a state yet it's debatable whether this example even counts as an invasion of the United States. Legally speaking, the United States proper ''was'' invaded because the Alaska Territory was ''incorporated'', meaning that under U.S. law it was part of the United States proper. On the other hand, territories like Guam and the Philippines were not, because they were ''unincorporated''.
* It should be noted that none of these were an attempt to conquer American territory:
** The South was where nearly all the fighting of the [[UsefulNotes/TheAmericanCivilWar US's Civil War]] was taking place, and General Lee calculated that if his forces could go on the offensive and defeat the Federal Government's armies in a decisive battle (before they had time to bring their massive resource-superiority to bear), they would push for a peace settlement and acknowledge the Confederacy's secession. Lee's move was also motivated by a chronic food supply problem -- Southern plantation holders were [[CripplingOverspecialization so hung up on growing cotton that there wasn't enough fertile land set aside for food crops]], while agriculture in the North was mainly foodstuffs like wheat and corn.
** Pancho Villa's army was so starved of supplies that he was willing to trespass into the US to get them in what was effectively a glorified supply-run.
** The occupation of the Aleutians was done to divert attention from the Imperial Navy's real objective, the US Naval base at Midway (it failed in this because the US's intelligence corps had cracked the Imperial Navy's radio codes/cyphers and knew exactly what was going to happen). It was also an example of ExecutiveMeddling, since Admiral Yamamoto's plan for Midway wasn't to capture the base but to draw the American aircraft carriers into a trap where they could be sunk (the cracked Japanese codes meant that the US Navy was instead able to reverse the trap). A diversionary operation in the Aleutians contradicted the entire point of the plan, but it happened anyway just because the higher-ups thought it was a good idea.

!!! Projected invasions:
* At the end of the 19th century, the German General Staff drew up [[http://europeanhistory.about.com/library/weekly/aa050902a.htm a planned invasion of the US eastern seaboard]]. (See the entry on ''1901'' in "Literature" above.) It is, however, the responsibility of a General Staff to draw up plans for ''every'' eventuality. The Austro-Hungarian General Staff was so paranoid it actually drew up plans for a war with Italy - ''while they were still allies'' (but not Germany, since such a war was seen as being as unlikely as it was unwinnable).
* The replacement of the population of the Soviet Union with ethnic Germans, and acquisition of the region's raw resources was a key goal of Nazi short-medium term planning, but was also key to gaining the requisite superiority over the USA in the medium-term (the 1960s-80s or so). The leadership of UsefulNotes/NaziGermany (and particularly Hitler) were almost certainly intending to take the USA on in the medium-long-term given their rhetoric, privately-held beliefs, and inordinate amounts of attention given to acquiring military baeses in west Africa and central-southern America even before the invasion of the Soviet Union. In the long-long term, [[TakeOverTheWorld betrayal of their ally Japan is almost universally accepted as an inevitable outcome of Nazi victory]] given the fundamental unacceptability of a non-European ethnicity controlling a major world power.
** Technically the Nazis did manage to invade part of North America when they set up monitoring stations in Greenland after seizing Denmark. Fifteen Greenlanders on dogsleds, 'the Sledge Patrol', shut down the monitoring stations and that was that. Although the closest the Nazis ever got to shooting at the American mainland was U-boats harassing USN shipping in American waters.
* According to a [[http://techconex.com/tcblog/2008/04/05/mapping-world-war-iii-soviet-global-invasion-routes/ 1987 Department of Defense plan]] that projected the path of a Soviet invasion in case of WorldWarIII, Alaska would have been invaded from across the Bering Sea in order to draw American forces away from Japan and the rest of Asia.
* In 1917, the German Foreign Office unilaterally (without the approval of the Reichstag or the OHL, both of whom thought it was a ''colossally'' stupid idea) offered Venustiano Carranza aid to invade the US with the promise that Mexico would [[MexicoCalledTheyWantTexasBack take back the land that was taken in 1848]] after the Mexican-American War (read: half of Mexico). They didn't do this for a very great number of reasons, not least a little something called UsefulNotes/TheMexicanRevolution. It didn't help that the Foreign Office had told the German embassy in Mexico to deliver this offer to them... using the trans-Atlantic telegraph line. Which was owned, and all its traffic monitored, by the British. WhatAnIdiot, indeed.
** The other problem was sheer logistics - Mexico was quite outgunned compared with the US (especially if the latter mobilized its resources for war, which an attack would certainly cause), and the only power that would have capable of giving Mexico the arms and aid needed to even consider invasion of the US was ''the US itself'' (anything from Europe would have been cut off by the US Navy).
* [[http://en.wikipedia.org/wiki/Defence_Scheme_No._1 Defense Scheme No. 1]] was a Canadian plan formed in 1921 to invade the United States if there was evidence a US invasion of Canada was imminent. It would have called for four avenues of attack (the Pacific Northwest, Minnesota, Upstate New York, and Maine) in an attempt to throw US forces off balance (given the large numerical advantage the US would have) before pulling back and destroying bridges, buying time for reinforcements from Britain to arrive. Amusingly, those reinforcements would never arrive in such an event - the Royal Navy at the time believed that in a hypothetical war with the US, Canada would be impossible to defend and so would not have sent significant reinforcements. The plan (as well as the corresponding American [[http://en.wikipedia.org/wiki/War_Plan_Red War Plan Red]] formed in 1930) was largely academic in nature, though War Plan Red's declassification in 1974 caused a stir in US-Canadian relations.
!!! Current situation:
* An invasion of the US as things stand now at the time of typing would be very difficult, as the US has the second-largest military in the world, with 1.5 million active personnel, including the largest air force and navy (fairly important when its main rivals are overseas), and spends more on its military than most of the planet's other major nation-states combined (and that's ''after'' budget cuts; less than a decade ago the United States spent more on its military than ''all'' other nations combined). What's more, the country's [[AmericanGunPolitics liberal gun laws]] would ensure no end of well-armed partisans to trouble an occupying force.
** During UsefulNotes/WorldWarII, Admiral Yamamoto of UsefulNotes/ImperialJapan knew war with the US was going to end in disaster because of the country's vastly greater economic strength - the USA's GDP being ten times that of the Empire's. A couple of quotes from him on the issue:
--->"You cannot invade the mainland United States. There would be a rifle behind each blade of grass." '''(Note that this quote may be a case of BeamMeUpScotty, as the record over whether or not [[https://www.thetruthaboutguns.com/2016/12/johannes-paulsen/thought-americans-december-7th/ Yamamoto actually said this is unclear]]. Regardless, it makes a good point.)'''
--->"Should hostilities once break out between Japan and the United States, it is not enough that we take Guam and the Philippines, nor even Hawaii and San Francisco. We would have to march into Washington and sign the treaty in the White House. I wonder if our politicians (who speak so lightly of a Japanese-American war) have confidence as to the outcome and are prepared to make the necessary sacrifices?"
** Unfortunately, the quote above was taken (with the second and third sentences removed) as a proclamation that he would do this trope instead of as a warning that this wouldn't end well.
** Not to mention the fact that the US also has [[UsefulNotes/AmericanLawEnforcement over 800,000 federal, state, and local sworn law enforcement personnel]], an increasing number of which are equipped with paramilitary weapons, equipment, and training (again, [[AmericanGunPolitics US gun laws]] play a factor here), and all of which would have much better local knowledge of their own jurisdictions than foreign invaders (critical in partisan operations). By itself, if they were their own army, the total number of American cops would be the sixth largest in the world, between North Korea's and Israel's.
** Along with much of the above, there's also the fact that the United States is a vast, mostly urbanized, geographically diverse country flanked by two major oceans and superior interior lines. The country's sheer size would work against an invading force, though the country's highly-developed infrastructure could be made to work for them as well as for a defending force. Invading the continental United States would be ''slightly'' more forgiving than invading Russia. ''Slightly.''
** It can probably best be summed up that Invading the US has all the problems of invading Russia and/or China (Huge size, large population), with the additional difficulty of supporting said invasion across 3000 (Atlantic) to 6000 (Pacific) miles of ocean. No nation on Earth has anywhere near the seaborne transport capacity to bring a sufficiently large army across an ocean to invade the United States, even ignoring the fact that they'd need to keep those ships from being sunk by the US Navy and Air Force.
*** To make things worse than dealing with the US military for an invader: US civilians collectively own 300 million+ firearms. There is more than one civilian owned firearm for every American adult.
** And, to top it all off, the United States is a NATO member, has a number of unilateral defense treaties with other nations, and is in general much more popular than China, India, or Russia, which are the only three countries that could fend off a conventional land invasion by the USA (though not a determined naval and air campaign). Anyone looking to resist an attack by the USA would have few, if any, allies to call upon. Anyone looking to invade could well see much of the developed world in the USA's side. For now, an invasion of the USA is a 'pipe dream'.
* The United States is also economically intertwined with most of the rest of the world. Any potential invader knows that, for example, cutting off American agriculture to the rest of the world will likely result in food riots, quite possibly within their own countries. China, especially, would have to worry about what would become of their economy without the U.S. to sell to, meaning they'd be far more likely to enter the war on the side of the U.S. than to be one of the invaders.
** This is, however, one of the reasons China has been so active in developing relations with other countries like in Africa: other markets to sell their products to means less reliance on the US buying their stuff, which means more flexibility in their foreign policy. Even then, it's still unlikely China might attempt to even invade Hawaii due to the myriad number of other obstacles, but they may be more willing to take a more firm stance in other issues like, say, Taiwan.
* Lastly, none of the countries commonly depicted as invading are really interested in invading the USA as all planners view the logistics of it as sheer impossibility. In the Cold War, both the USA and the USSR dismissed any serious idea of invading and seizing the other's territory as thoroughly impractical, something which still holds today. They may have drawn up some plans as a thought exercise (a military planner has to come up with plans for damn near anything, even ones impossible to pull off), but never really thought they could do it without bringing forth TheEndOfTheWorldAsWeKnowIt from all the nukes it would send flying.