Game Breaker / Hearts of Iron

All the little things which can turn taking over the world in Hearts of Iron trival. Some of them apply to all instalments, some to specific parts.
  • The United States is pretty much considered "easy mode", and for good reason, particularly in III. They start off with the largest amount of IC in the game, the biggest knowledge pool, and a very powerful navy — and it only gets bigger and meaner as the game progresses. And then there's the obvious fact that the Axis can't hit the American mainland because of geographical separation, and even if they managed to get a few ships to the coastal areas, they'd get massacred. The only weaknesses the US has is that, starting out, they have a weak army and air force and low technology, so using that knowledge pool is essential to catch up with the rest of the world. But by '45, the US will almost always be in a dominant position, as the only other economic powerhouse, the USSR, will have almost certainly spent a lot of resources and taken a lot of damage fighting Germany. All of this isn't just Truth in Television, it's actually toned down compared to how powerful the US economy was in World War II, complete with the fact that, by the end, the USSR was the only nation that could hope to compete with them. Later expansions make the US even more broken, with For The Motherland actually allowing the US to get a massive manpower boost once "The Day of Infamy" event triggers, which not only adds 400+ manpower instantly to their pool, but also revokes The New Deal (which imposed a manpower penalty on the USA) and gives a 25% bonus to manpower growth for a couple of years.
  • Starting from II, engineer brigades are one of the most useful things you can equip your units with, doubly so if fighting a defensive war. Engineers are relatively cheap addition to your troops and cost small amount of fuel from the start, but in the same time they considerably increase defense capabilities of units they support, increase dig-in cap, remove or at least seriously decrease penalties for fighting in otherwise horrible terrain, bypass fortification bonuses, considerably decrease amphibious assault penalty, allow effortless river-crossing and - at least in II - increase the speed of unit they accompany by 1. In case of III, they start with speed of 8 - which is enough to keep up with medium tanks and motorized infantry without slowing them down. In II, later models also start to gradually decrease softness value of units they accompany, which means at least part of the damage that normally hurts infantry will be ignored.
    • IV changed Engineers from being a brigade to a support company, but made them into a support company that every single possible unit can benefit from having due to their all around bonuses and cheap price. While a lot of support companies are situational, Engineers are a mainstay regardless of anything.
  • In II, producing Garrisons with Military Police detachments can completely negate partisans. It might not sound impressive, but Garrisons in such configuration can outperforming presence of few divisions in a province and quite efficiently spill the control to the neighbor ones. This was nerfed in III, where conquered or controlled foreign land always has minimal partisan rating, slowly decreasing over the course of 20 years, which is more than the game lasts.
  • Practical knowledge from III. It replaced already powerful gearing bonus from II, but gearing bonus required continuous production of the same series of units. Practical knowledge is just accumulated with each finished building or unit. And it not only decrease the time and costs required to finish production, but also affected related research by really large margin.
    • One of the most gamey tactics for industrialization is based on building relatively cheap infrastructure (which is always useful to move supplies and units), thus quickly gaining large amount of Construction practical knowledge, making factories much cheaper and finished in few weeks instead of over a year... and they will further fuel the practical knowledge, snowballing the bonus further. Or just building suddenly cheap and fast forts. USA pulling this goes from already powerful to virtually unbeatable due to having bigger industry than all other Great Powers combined
    • Preparing anti-air research has similar, and utterly accidental, effect. By building just a handful of provincial AA defenses, practical knowledge for artillery skyrockets, greatly decreasing construction and research of all types and sorts of heavy guns, including those from tanks.
    • Building vanilla heavy cruiser - without absolutely any improvements on board - takes roughly 6 months and costs nothing. And even if seriously underequipped, it will still do fine up until about '41 mark or can be used to hunt down convoys with no fear from destroyers. But the real kicker is about how it provides capital ship practical knowledge and a lot of it, which can in turn greatly speed-up construction of the real deal: battleships and battlecruisers. Normally they can take up to three years to finish. Japan can gain this way mad amount of practical knowledge for capital ships, thus entering Pacific War with numerous and modern battleships, utterly wrecking US Navy or even standing a chance against player-controlled America.
      • Germans start in '36 with few of their pocket cruisers almost finished - which makes construction of not two, but entire series of Bismarck battleships even more tempting.
  • Naval Bombers in II, to the point that many players constructed Naval Bombers exclusively instead of naval fleets. Paradox tried to combat this problem in later patches by (unrealistically) requiring other ships to detect enemy fleet before Naval Bombers could engage them. While not as useful as in II, even in III they are still very powerful units against enemy fleets, especially convoys. And depending on size and shape of the air theatre toward location of naval bases, they can potentially sink half of enemy ships within a month of intense bombing in IV.
  • Close air support units, especially in II and III. They are really good at their job (directly hitting infantry and tanks) and can keep up on their own in the air, while other bombers require escorts and general air superiority. But what really makes them game-breakingly powerful is the ability to cause damage to retreating units. Normally when unit is retreating, it can't be hurt with anything, short of nukes. CAS units can keep pounding such units and do so with complete impunity. So when unit finally reaches nearby province, it can loose up to third of its strength, making it an easy pick for ground units... and the bombardment can continue. Also, in II, most of CAS doctrines are accessible by '38 mark, making their missions roughly twice as efficient before the war even breaks out.
  • Humble drop tanks from III. While all fuel tank improvements make aircraft easier to pick and slows them down, drop tanks can go as far as double range of certain types of planes, with minimal drawbacks. This is extremely useful when fighting in Russia (where airfields are few and sparse) and in the Pacific (where you only have a handful of islands and carries as possible landing spots) and can pretty much turn the tide of war all by itself once implemented.
  • Strategic rockets, especially after developing V2 or stand-ins for it. Unlike strategic bombers, rockets are extremely cheap, and more importantly, fast to construct. So what if they are one-use-only, if singe rocket can achieve in one hour more than strategic bomber in a month. By the end of the month, you will produce another two. It is entirely possible to produce enough rockets to absolutely crash entire nations, especially in II and III. In fact, in II rockets are often banned in multiplayer, because with loss of IC, players also lose slots for their tech-teams, which alone can cripple a country without any way to regain lost time. If played right, a handful of rockets can turn England into rubble, while the infrastructure will be so damaged it will take at least few months to get the country up and going. And when the industry is down, no units are produced. At all.
    • In IV, due to how repairs are reworked, it's entirely possible to cripple AI forever. AI has a tendency to convert as much factories as possible into the war material ones. Repairs are done with use of civilian factories. And AI will keep only the minimal required amount, so in case of heavy bombardment, it might take years to rebuild, while also lacking resources to retool some factories into civilian ones.
  • Convoy raiding and strategic bombardment of any type is this in certain versions of III (it was nerfed quickly with patches). The national unity can be decreased with extensive destruction of merchant marine and continuous bombing. In fact, it's possible to bomb Britain hard enough for it to surrender, while not doing any invasion on the Islands.
    • If done right, convoy raiding can completely collapse AI's ability to conduct war. It was proven time and again a continuous and extensive convoy raiding will render British Empire defenseless, as troops stationed in the colonies will be extremely undersupplied, making them easy pick for Japan and even Italy. Meanwhile, a large stockpile of resources will wait in colonial ports, allowing conquerors to take them over and use for their own industry.
  • Speaking of submarines - torpedoes research in Darkest Hour and III can turn already powerful submarines (if well-used) to terror of the sea, especially if rushing the tech. With proper research, submarines can get so powerful, a single unit of them will be capable of what normally takes three. And the smaller the size of submarine stack, the harder it is to detect them, making them even more efficient. But more importantly, the torpedoes increase general hitting power of subs (and also increase their attack range in Darkest Hour), so it's possible to take down destroyers with ease - while normally destroyers are designated submarine hunters.
  • Paratroopers:
    • Dropping behind enemy lines can in certain cases win a war within a day or so. AI tends to focus all the units on the borders, rarely keeping anything stationed in the interior, aside maybe capital and extremely important strategic locations. Which means most of the provinces containing victory points are left unguarded. And they usually also contain factories. Few well-timed drops can take out most of enemy industry and victory points, either shortening the war considerably or outright winning it. Suicide mission for capital is also one of pretty good strategies.
    • If capital is taken, entire resource base is lost and all supply lines are cut. This is considered equal to cheating and usually banned in multiplayer. But outside outright gamey strategies, sudden para-drop can cause massive disruption of enemy supplies and quick encirclement of large amount of troops.
    • Para-dropping into nearby provinces is also very helpful while conducting amphibious assault, as it prevents enemy from sending reinforcements, while causing encirclement and double envelopment of troops guarding shore - they have to deal with landing party and paratroopers in the same time, while being cut out of supplies, which can cause penalties so high their defense percentage might end up reduced to single digit.
    • If the drop zone happens to have an airport, you can resupply your paratroopers the moment they take over the province, or, more importantly, pick them up and hit another target, this time deep behind enemy lines. And your bombers and dogfighters just gained a staging ground, while enemy is denied landing zone.
    • If by chance any air wing is stationed in airport when a province it taken, all the planes are instantly destroyed. Sudden drop behind lines on unguarded airfield, before the planes can take off, can absolutely wreck enemy air capabilities.
    • That being said however, Paratroops come with some very significant drawbacks. Paratroops are reliant on transport plains that are the most expensive unit type in the entire game and this requires for you to both research and build two unit types at the same time. Transport planes are also completely defenseless requiring you to have a proper fighter escort to protect them during their missions. Paratroops are also the third weakest Infantry unit in the game after Garrison and Militia, meaning that if they face anything stronger than these units in equal or superior numbers, they are going to be overwhelmed in a very short order. Since Paratroopers start out most of their missions being encircled, loosing a fight will almost certainly mean that you will loose the entire Paratroop division you just deployed, requiring you to build an entirely new one.
  • In III, using wars with minor countries to let you pass Total Economic Mobilization and Service By Requirement laws lets you swell your IC and manpower by a hilarious degree before World War II starts. Especially bad if you're a major power, and just ignore the "war" while you build up a huge number of divisions, planes, and ships. This got so bad that for the Their Finest Hour expansion, a special restriction was set where those laws could only be passed if the enemy you faced had a minimum of half your IC, otherwise you're stuck with just War Economy and Three-Year-Draft.
  • Earlier editions of III turned your intelligence apparatus into one of these when used properly. The "Sabotage Production" mission, when coupled with "Counterintelligence" to eliminate enemy domestic spies, enabled allowed you to utterly cripple an enemy's industrial capacity, to the point that, for example, Germany would take months to conquer Poland and would get stonewalled in France, leaving them ripe for an American or Soviet attack. Later expansions removed the ability to sabotage production.
  • Playing fascist is seen as this in Hearts of Iron 4. You can get yourself ready for war faster, can have a larger manpower pool, can invade other countries and puppet them easily, and can have numerous industry-boosting attributes. Democracies and Communism are seen as inferior, compared to it.
  • Also in IV, all the artillery support brigades you can attatch to divisions. While they are just a single brigade of given type, they still provide additional punch to your unit, while not following rules of standard artillery. Adding standard towed artillery brigade(s) decrease speed of the unit and costs a lot of resources. Support units don't have this problem, so you can attatch artillery and AT guns to your light tank brigade without slowing it down, while almost doubling its fighting capabilities, at almost no price to that. They also don't have any terrain penalties that are a nightmare for artillery divisions, thus Mountaineers, Paratroopers and Marines can gain additional punch, without any trade-off
  • Recon brigade in IV. They are the only way to increase recon value of a division. During combat, the unit with higher recon, regardless if defender or attacker, has combat initiative. The higher the recon, the bigger the chance for commander to pick the best tactic for given moment. And if your recon is higher than enemy's, you gain reroll for tactics, always being able to counter their commander. Since it's cheap and accessible from the start, it's a must-have for almost everyone.
  • Heavy Tanks in IV. Putting a single Heavy Tank battalion even on regular infantry divisions will give such a massive boost to their armor, that the AI can't figure any way to counter it, effectively always giving you a massive 50% increase to damage and defense.
    • And self-propelled artillery guns mounted on heavy tank chassis takes it a step further, as they provide insane amount of soft attack and decent hard attack, while retaining all the other benefits of heavy tank batallion. They are so powerful, most of guides for long-lasting wars with bigger powers than own country call for using those SP guns. While they are relatively expensive to make and need to be researched first, a single battalion of most primitive model has bigger punch than most end-game artillery pieces. They are so broken, they can be even used against unsuspecting human-controlled army in multiplayer. And if another battalion of anti-tank version is added...
  • Maintenance company from IV allows to completely ignore reliability stat of land units. Normally, you can spend army experience to make better models of given equipment, but it almost always decrease their reliability, so you have to spend a lot of experience on reliability too, or the equipment will be prone to breaks, jams and other mechanical problems (think German Tiger and Panther tanks). Maintenance company starts with +20% to reliability (and non-modified equipment has reliability of 80%) and only goes higher from there, so you can completely ignore reliability of your tanks and artillery, saving huge amounts of experience, while enjoying all the benefits of better models.
  • Variants in IV, in two distinctive flavours.
    • As the war goes on, getting mad amount of land experience is very easy. All it takes is fighting. Same goes with air combat, where it's even gained for bombing empty provinces - after all there is no longer fuel or supplies in the game, so you are not spending any resources on such bombardment. Thus its very easy to design new variants of already researched models. It's costly in terms of experience needed, but well-made variant is roughtly 80% more powerful than next model in the research tree (this is especially true for tanks). Not only that, but variants have two massive benefits over completely new models. They don't get the penalty for switching production lines (since it's the same model with just minor modifications) and, which is much more important, they can be fielded few years before its feasable to start researching next model, giving a huge edge much earlier. If done right and jumping a gun a bit, its entirely possible to synergise research of new, crucial model with making a variant out of it from the get-go, thus producing superior unit without any delays.
    • Certain countries, like Japan or the USA, gain large amount of naval experience (which is the hardest to gain, as it requires a lot of naval combat, which is rare and deadly for really expensive units taking forever to replace once lost) from their political tree. Unlike land units and airplanes, the year gap between naval models is so big and the time required to produce new models so long, it's simply impossible to produce some of them before the war will be already over, thus making most fleets composed of units from '40 and '41. Variants in this case are thus of even greater importance. Hangar for '40 carrier can only take 70 planes inside? A maxed-out variant can take in 96. And if researched with shipyard specialising in carriers, then 114. The kicker? Next model, from '44 (and which won't be build before mid-to-late '45) has hangar in size of 85 (106 if researched by carrier specialist). So for five years you are taking full advantage of unit that won't even get obsolete and can be still produced without need of researching new one.
  • With Death or Dishonor DLC for IV, out of all countries, Yugoslavia and Czechoslovakia, both minor states that can barely hold ground (and Czechoslovakia isn't even there anymore in '39 start), turn into absurdly powerful countries even without any gamey tactics and tricks. Czechoslovakia can quickly turn into a fortress almost impossible to pierce, with all borders covered with high level forts and recruitment pool increased enough to man all those forts, stalling any possible advances forever over hard terrain and numerous bunkers. Yugoslavia almost doubles its resource (it's one of the main exporters of aluminium from the start and gains more of it) and industrial base, while gaining additional research and military bonuses (and since it has much more resources than it will need for a looong while, they can be easily traded for free access to foreign civilian factories, allowing to quickly build up own industry) and basically is stronger than neighbouring Italy in the end of national focuses. This is openly mocked by fandom for utter lack of balance, ground in history or just basic geological reality.
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