Vestigial Empire / Fan Works

• In the Gene Catlow fanfic The Basalt City Chronicles, the Empire of Smilodons once ruled an empire spanning from Burma in the southwest and Chile in the southeast, to the Bering Strait in the north. They're now down to a group of islands in the Bering Strait, though they've still got plenty of their ancient national treasure...
• In the 1983: Doomsday Stories for Hetalia: Axis Powers, there are nods to Austria still feeling this despite being one of the core members of the Alpine Confederation, though with painful justification. Having been reduced to a neutral Cold War buffer by the time Doomsday came, he was powerless to stop the bloodshed and save Hungary, which would come to haunt him in the years to come.
• Strengths of the Heart and Mind: Long ago, the Dark Bonneter-led Corona Empire ruled a large portion of the world. Eventually, rebellion broke out and their territory was reduced to the small town of Varia in the Cap Kingdom. Most Dark Bonneters today are content with their small settlements, but not Conicus.
• A Brief History of Equestria: By the time of Hearth's Warming, this is the fate that had befallen both the Mongrellian Horde and the Griffin Empire, which had collapsed in ways similar to the Mongols and the Roman Empire, respectively (fittingly, as they are presented as Expies of those nations).
• The Griffon Empire of RainbowDoubleDash's Lunaverse offers an interesting twist on this trope. Instead of the Empire shrinking, it shattered into dozens of feuding kingdoms. Collectively, the Kingdoms still control about as much territory as the old Empire, and represent comparable amounts of military and economic power, but it would take something truly drastic to unify them again.
• Shattered Empire focuses on the Imperial Remnant of Star Wars Legends and mentioned below in the literature section as the main protagonist. Like in the source material they used to be Palpatine galaxy spanning Empire before being reduced to a mere eight sectors by the New Republic. They however seek to restore their status to how it was during Palpatine's reign.
• The Vagaari Miskarate (formerly called the Vagaari Empire) in Star Wars: Ascendancy. Prior to Thrawn's exile, the Vagaari were a vast slave empire that threatened the Chiss Ascendancy, controlled a large portion of the Unknown Regions, and fielded an impressive fleet of warships hundreds strong. By the time the Chimaera encounters them, the Vagaari's repeated wars with the Chiss, combined with Miskara Pesh's incompetent leadership, have seen them reduced to wandering the stars aboard a few dozen ramshackle starships in constant states of disrepair. This is further reflected by the Miskara's throne room, which is brightly lit to create an illusion of opulence and has aged walls covered with rust. After the Voidlord makes off with all the Vagaari's treasure ships, they are left in an even sorrier state as they suffer from chronic resource shortages and their entire way of life is said to be on the verge of becoming unsustainable.
• Earth's Alien History has a couple of examples:
  • The Gamilians are one of space-faring humanity's first major enemies, but after losing the Mekon War, they retreat and end up fracturing back into nation-states. Then they get invaded by the Reapers and rescued by the Romulans, to whom they become a Voluntary Vassal.
  • The Citadel races. War with the Space Pirates heavily damage their economy and infrastructure, then the Mekon and his allies invade, annexing the Batarians, damaging many of their key worlds, and detonating an Omega particle near the Citadel itself which makes FTL travel to the station difficult. They recover somewhat by offering seats on the Council to all member races, but after the Five Galaxies War, the Quarians and Krogans defect to TeTO, weakening them again. Then the Reaper War hits them even harder, with Pavalan being destroyed and the Turian Hierarchy effectively ceasing to exist. Following the war's end, the now terminally weakened Council eventually disbands out of necessity, the various members joining TeTO or the Pact of the Raptor, leaving just the Citadel itself, independent as essentially a city-state.
  • The Centauri Republic is almost completely dismantled by the Crusader Pact, with Centauri Prime and its main colony being destroyed and them then being forced to abandon all colonies that previously belonged to other races. They rebuild from this remnant, however, becoming a sort of space Switzerland in later years.
  • TeTO becomes this in the Mirror Universe after losing the Mekon War. Not only does the detonation of the Omega particles destroying their ability to use FTL in the heart of their territory heavily cripple them, but the Race abandon the alliance in order to avoid continuing conflict with the Mekon and his allies, significantly weakening them further.
  • The Klingon Empire goes into decline in the years following the Reaper War, due to being economically and technologically outmatched by all the other powers, with the societal tensions following the Praxis War not helping matters any. Following their second civil war, they're reduced to the Klingon Republic (a TeTO protectorate) and the Empire of Kahless (a hermit kingdom), the latter of which loses even more territory when the Unas Klingons rebel and split off to form the Pure Klingon Empire.
  • After the Goa'uld War, the Goa'uld Empire is reduced to just Bastet's domain, wherein Bastet herself is just a powerless figurehead to the Ka'tee movement, as well as a small group of System Lords and their remaining forces led by Ba'al who flee to the Delta Quadrant to start over.
• In the Discworld of A.A. Pessimal, Ankh-Morpork is explicitly an Expy of post-Imperial Britain. Due to the great conquering General Tacticus, its former colonies are everywhere. It is perhaps easiest to list those parts of the Central Continent that were not part of the Empire and which served as its rivals; Quirm being most noteable and which, in fact, left an Imperial footprint of its own. There's also the hinterlands of Far Uberwald; left to its own devices because Tacticus expressly refused to get embroiled in fighting a war with that country in the middle of one of its winters. After Tacticus, the Empire declined. today its last remnants are the Shires, the hinterland immediately surrounding ankh-Morpork, and places like the Guano Islands - which in defiance of the usual expectations, actually fought a long and bitter War of Dependence to remain an Ankh-Morporkian colony as they saw their standard of living going right down if the Morporkians pulled out - and therefore didn't have to financially support them any more. Ankh-Morpork's empire also had the usual more conventional wars of Independence at its ending; Rimwards Howondaland fought its, err, Boor War. Aceria also took the opportunity to fight a war by twilight's last gleaming and the Dawn's early light. The other half of Aceria remained broadly loyal, however and got its independence peacably.
• With Hordak no longer leading the Etherian Horde, some members of the Horde decided to occupy Erelandia (the Fright Zone having been given back to Scorpia's family) as the New Horde She-Ra: In The Wake under a new leader. While the General sees this as the dawn of a new era for the Horde, Hordak calls the General a carrion feeder trying to make a name for themself using his accomplishments.
• This was Nanoha's justification for refusing to claim her birthright as Princess Serenity in White Devil of the Moon: by the time she and the other Senshi had awakened, the Moon Kingdom was in ruins from the Dark Kingdom's attack, and aside from those directly involved in the Dark Kingdom case, there was no one that even knew the Moon Kingdom had existed, except a few scholars that mentioned it in papers about these type of countries. Luna, who wants nothing more than to make the Moon Kingdom a Resurgent Empire, doesn't take it well.
• Chasing Dragons: By the end of the Fourth Slave War, Lys is reduced to a shell of itself — its mainland territories are ceded to Myr, it's forced to cut contact with Volantis and Slaver's Bay, limit the size of its navy, and start the process of manumission for the slaves still living on its remaining islands, and is also made to accept the presence of Braavosi troops in order to enforce all of these rules, effectively making them a Puppet State.
• Invader Zim: A Bad Thing Never Ends: The Irkens have been in rapid decline ever since the Tallest stupidly flew into the Florpus and took the Armada with them, leaving the Empire's military undermanned and their galactic communications cut off. Planets are rebelling, the Planet Jackers have broken their peace treaty and are stealing worlds from them, and even the Resisty have proven themselves to be an actual threat.
• Word of God says that the Empire of Smilodons in The Basalt City Chronicles once ruled western coasts of both Americas, and even the North-Eastern coast of Asia, but is now relegated to a few islands off Alaska.