The standard mode of territorial expansion of The Empire is steamrolling all opposition in the targeted neighbor land and claiming it as its new vassal state-slash-province. However, some places and people prove too troublesome to conquer and the empire leaves them alone. And then some of them decide to swear allegiance to The Emperor, anyway, for the benefits that a large centralized state brings with it—minus all the inconveniences of being forcefully subjugated by a foreign invader.
Characters coming from such a region commonly enjoy privileges and personal freedoms that the rest of The Empire's citizens (except the metropolitans) no longer have.
Compare/contrast Hegemonic Empire, where The Empire doesn't expand through conquest, but instead incites other states to voluntarily become its vassals by peaceful means (in effect, making this trope a norm instead of exception). Compare also Puppet State, Les Collaborateurs, and The Quisling. Vichy Earth may or may not be an example as well.
- Crest of the Stars: Happened in the backstory to planet Midgrat. The planet was independent and among the more powerful independent nations, but they saw that the various galactic superpowers were making such a position untenable, so they opted to join with one. Each superpower sent delegates to entice them, and in the end they chose to join the Abh Empire, mainly because the Abh don't tend to interfere with local cultures.
- The Equestrian province of Latigo in RainbowDoubleDash's Lunaverse. Unlike most of the regions that Equestria has annexed over the years, it came in more or less voluntarily and under two major conditions. Firstly, its ruling line married into House Starlight, the unicorn viceroyalty (there are three viceroyalties, one from each of the three pony tribes. Only Princess Luna herself ranks higher in Equestria's nobility). Secondly, it was annexed as a unit, becoming one of Equestria's largest and most prosperous provinces.
- In Earth's Alien History, both the Gamilans and the Orions become client states of the Romulans after they save them from the Reapers.
- A Song of Ice and Fire:
- Dorne was never permanently conquered (although it was briefly occupied) by the Targaryens, despite numerous attempts. Instead, they joined the Targaryen realm via dynastic marriage, conferring to them a special standing ever since. The family motto of their ruling house reflects this: "Unbowed, Unbent, Unbroken".
- Torrhen Stark, called "the King Who Knelt," surrendered to Aegon the Conqueror and joined the North to his new united kingdom after calling his banners and marching south only to meet an army half again as large as his, with three dragons besides.
- The Lord of Oldtown also voluntarily surrendered and pledged fealty to the Targaryens after the High Septon (think fantasy Pope) claimed to have seen a vision from the Crone (goddess of wisdom) that the city would burn otherwise. The High Septon then anointed and crowned Aegon as King of the Seven Kingdoms, although the Iron Islands were technically not yet conquered and Dorne wouldn't join for another hundred years.
- In Tribesmen of Gor, two powerful desert tribes, the Aretai and Kavars, have made virtually all the other tribes of the desert into vassals—except the Tajuks, who are independent. Because generations ago an Aretai warrior had saved a Tajuk's life, the Tajuks always ride into battle with the Aretai.
"The Tajuks are not actually a vassal tribe of the Aretai, though they ride with them. More than two hundred years ago a wandering Tajuk had been rescued in the desert by Aretai riders, who had treated him well, and had given him water and a [camel]. The man had found his way back to his own tents. Since that time the Tajuks had, whenever they heard the Aretai were gathering, and summoning tribes, come to ride with them. They had never been summoned by the Aretai, who had no right to do this, but they had never failed to come."
- In Reflections of Eterna, Kenalloa (the Fantasy Counterpart Culture of Spain) has a long history of independence and special status: its dukes joined the old Taligoya voluntarily, formally bowing to the Rakans, but never fully submitting to them (e.g. rejecting the Esperatian state religion). When the Rakans were overthrown by the Ollars, Duke Alva of Kenalloa sided with the latter, and his son basically became co-monarch later, cementing Kenalloa's special status within the new kingdom of Talig. Meanwhile, Duke Neumar was the last Rakan loyalist who wasn't re-subjugated by the Ollars: instead, Neumarinen was brought into Talig via dynastic marriage. Both it and Kenalloa have the legal right to secede from Talig at any time.
- In The Witchlands it's mentioned that the nations of Svoden and Portolla have allied with the Cartorran Empire to maintain some level of independence.
- Towards the end of Star Trek: Deep Space Nine, Cardassia agrees to become part of the Dominion. It's voluntary at first.
- Game of Thrones: Jon Snow is crowned King in the North at the end of season 6, but in season 7 he relinquishes his crown so he can become Warden of the North to Queen Daenerys Targaryen (whose dynasty claims kingship over all of Westeros), since there are far bigger problems they have to face than dynastic squabbles.
- In Star Wars: The Old Republic, the Chiss Ascendancy is unique among the subjects of the Sith Empire in that the Imperial military failed to capture its homeworld and gave up, but then the Chiss decided to join the Empire, anyway. This causes a lot of awkwardness, as the Empire is openly xenophobic, yet the non-human Chiss enjoy a lot of privileges that even the human residents of Sith-conquered planets don't.
- The Elder Scrolls:
- The province of Morrowind joined Tiber Septim's Third Tamrielic Empire voluntarily. Vivec (having recently lost two of the Tools of Kagrenac to Dagoth Ur and thus, the ability for the Tribunal to recharge their divinity) negotiated this status with Tiber Septim when Septim's forces threatened to invade. Vivec also offered the Numidium in trade, allowing Morrowind to keep many of its pre-Imperial privileges such as slavery (which was illegal elsewhere in the Empire), continued Great House rule (though with Imperial-styled ranks, titles, and conventions like a Puppet King added), and the freedom to worship within Tribunal Temple (though Morrowind had to allow free worship of other religions as well).
- Hammerfell re-joined the Septim Empire voluntarily — Hammerfell was originally conquered by Tiber Septim after the losing side of a civil war invited the Empire in, but after the Imperial provincial administration was overthrown by Redguard rebels, the new government elected to open negotiations with Septim to become vassals on fairer terms. (Much of this plays out during the series' Action-Adventure spinoff game, Redguard.)
- The 4th Era iteration of the Aldmeri Dominion (led by the facistic Thalmor) convinced the Khajiit of Elsweyr to join peacefully as vassals. It helped that the Dominion claimed credit for resolving a crisis with Nirn's two moons, which are sacred to the Khajiiti people.
- In Mount & Blade Warband, this is an alternative (faster, but more risky) way to become a landed vassal. Normally, you sign up to one of the kingdoms as a mercenary, then swear fealty as a landless vassal, then get a burned-down village, and generally wait a LONG time until you can receive a castle or a city on some conquered territory. With this method, you conquer a castle or a city as an unaligned warlord, proclaim your own kingdom and immediately ask one of the kings to accept you as their new subject. You will be accepted and keep the castle or city you conquered.
- In Crusader Kings and its sequel, it's possible for a holder of a lower-tier title (i.e. Count or Dukenote ) to swear fealty to a holder of a higher-tier title (i.e. Duke or King, respectively) without outside prompting. It's rare for it to happen outside of player control, however. Of course, higher ranked rulers can offer vassalization to any lower ranked independent ruler. And they're a bit more likely to accept if you've say, conquered all their neighbors with military force for example. I.e. a fan-favored "tutorial" scenario is to start out as one of the petty kings (count or duke-level) of Ireland, conquer half the others through the usual combination of fabricated claims, arranged marriages, and other casus belli, create the kingdom of Eire, and tell the remaining petty kings they'd be better off as vassals.
- Europa Universalis has a parallel mechanic, where a large nation allied to a much smaller one can turn the latter into a "vassal," granting the overlord nation a portion of its income, preventing either from opting out of wars which involve one, and allowing the large nation to diplomatically annex the small one after some time has passednote . Not all vassals are voluntary, however—it's not uncommon to force a small nation defeated in war to be the victor's vassal.
- In Sword of the Stars II: The Lords of Winter the "Protectorate" tech allows player empires to peacefully absorb independent worlds by treaty. But the Zuul instead intimidate worlds into offering them tribute, mostly slaves.
- The Civilization series introduces this in later installments:
- IV: The Beyond The Sword expansion introduced vassal states to the game, and in most cases a civilzation will only become another faction's vassal as part of a peace treaty to avoid being wiped out. But if you're strong enough, other leaders may occasionally ask to become your vassals if they're afraid of being conquered by another rival. This can make accepting them as vassals problematic - said rival may declare war on your vassal anyway and drag you into a conflict you don't want or aren't ready for, and of course if you wanted to do some conquering of your own, you're not allowed to declare war on your vassals.
- V retains this mechanic for NPC city-states, as you can curry favor with them to get them to ally with you (granting bonuses and any connected resources in their lands) by completing quests they offer. More benign ways include giving gifts, opening a trade route, building a road connecting them to your empire, spreading your religion, clearing out a nearby barbarian camp, and bullying their rivals note .
- In V the various late game "Ideologies" give a way to passively gain influence with city-states. Freedom probably has the closest to this trope, giving bonus influence per turn just by having an active trade route.
- Stellaris has a variety of ways to render other species subjects of your empire, for instance technologically uplifted species automatically become protectorates when they obtain FTL and vassals once their tech is on par with their patron.
- As of version 2.0 gaining a Casus Belli to subjugate another empire requires giving them the opportunity to peacefully become a vassal first. And on occasion a weaker empire will request to become a vassal of a stronger empire that might offer them protection.
- Mass Effect has the volus, who voluntarily became a client race of the Turian Hierarchy, offering their mercantile expertise in exchange for turian protection.
- Several small island nations in the Pacific have entered into such associations with other countries, mostly the US (e.g. Micronesia), Australia (e.g. Nauru) and New Zealand (e.g. the Cook Islands). While the precise details are usually sui generis, they often entail a common defense taken care of by the large nation and somewhat laxer immigration rules.
- During the Spanish conquest of Mexico several tributaries of the Aztec Empire sided with the Spaniards, converted to Catholicism, and became willing subjects of Spain. Figuring that it would be better than continuing to provide the Aztecs with Human Sacrifices.
- In the 13th century a few of Iceland's chiefs became vassals of the King of Norway, then they started a 44-year civil war that ended with the entire island becoming vassal to Norway.
- The Falkland Islands are a voluntary colony, even though this term is now not applied any more. In several referenda, overwhelming majorities of the inhabitants of the islands have voted to keep the status quo (being governed by Great Britain with some limited home rule) over any other alternative, including both independence and being annexed by Argentina. The fact that the islands have no economic activity besides tourism sheep and fisheries note and Argentina tried to conquer the islands - while governed by a brutal military dictatorship - may help in keeping the Kelpers loyal to the crown.
- Some US states were (or at least claimed to be) sovereign nations before becoming another star on the American flag.
- The most well known example is Texas, which broke away from Mexico and ultimately joined the US.
- Vermont also spent a short time as a self proclaimed independent Republic.
- Whether or not the weird limbo state of the current US territories, Puerto Rico being the most widely known of them, is entirely voluntary on their part is an interesting question best discussed elsewhere, but the fact of the matter is that some votes have been held on this, at least in Puerto Rico. There being more than two options, it is not all that easy to say what "the people" want. That being said, none of the referenda have been so conclusive as to provide a definitive basis for changing the status quo. They also show that of the various options, outright independence has for decades been one of the less-favored options (never getting more than about 10-15% in the polls) even though the federal government is unlikely to deny PR its independence if its people express a clear intent to take it. This means that for the foreseeable future, whatever situation the Puerto Rican people choose for themselves, it more or less falls into this trope for lack of any other classification.
- The Commonwealth realms are the closest major 21st century countries come to feudalism. Basically Australia, Canada, New Zealand and a couple of others have a monarch an ocean away as their head of state who comes to visit only once every couple of years and they are apparently quite happy with that. Some countries have even tried joining the Commonwealth (though not the Commonwealth realms, a country can be member of the Commonwealth without having the English monarch as head of state) despite never having been a British/English/Scottish colony. The "protection" thing mentioned above may play a role in this as may "soft power" and development aid.
- The Costa Rican province of Guanacaste invokes this trope in its coat of arms where the official motto reads "De la patria por nuestra voluntad" - of the fatherland by our [own] will. This is due to Guanacaste being disputed in the 19th century between the emerging nations of Costa Rica and Nicaragua that arose from the rubble of the United Provinces of Central America. Ultimately Guanacaste ended up with Costa Rica and the Rio San Juan, which separates the two countries in the East was awarded to Nicaragua in its entirety. This of course solved all border disputes between the two countries... Nah, just kidding. Since 2000 alone the two countries have clashed over a road, a few cut down trees, an unpopulated island that google maps claimed to be Nicaraguan before claiming it was Costa Rican and Nicaraguan President Ortega even questioned whether Guanacaste should belong to Costa Rica.
- If you subscribe to the belief that the European Union is a Hegemonic Empire (which would then raise the question who the hegemon is, but let's not get sidetracked), each member would be a Voluntary Vassal, because they joined entirely voluntarily and as the referendum now known as "Brexit" has shown, there is an open door if anyone wants to leave. Playing the trope even straighter are a handful of non-members of the EU (Iceland, Switzerland and Norway mostly) who of their own free choosing adopt a lot of EU legislation in which they have no say in exchange for free trade and open borders as well as some considerations on the fields of fishery (Norway and Iceland) and/or foreign policy (Switzerland).
- Before World War II, Nazi Germany invaded and annexed Austria in a process known as Anschluss or "joining." In a subsequent plebiscite, the vast majority of Austrians voted in favor of Anschluss. This was undoubtedly due in large part to pressure on the populace by Those Wacky Nazis, but many Austrians did genuinely support union with Germany (though how many is still a matter of debate).
- Much of The Raj consisted of "princely states" led by native rulers (with titles like nawab, nizam, sultan, raja, maharaja, etc.) who joined The British Empire in exchange for protection and cementation of their local authority.