After the Fourth Crusaders destroyed Constantinople in 1204, they set up a new government. It is known as the Latin Empire of Constantinople, but in Latin, it was Imperium Romaniae, so we could translate it as the Empire of Romania. The Crusaders elected Baldwin Count of Flanders to become the new Emperor.
According to the treaty signed by all the Crusade leaders, the new Emperor Baldwin only controlled a quarter of the city. Venice controlled another 3/8, a section that included Hagia Sophia Basilica. The remaining 3/8 went to other Crusade leaders. Venice also gained control of 3/8 of the Empire, on paper.
In reality, Venice took over Crete and some other islands. The Crusaders divided Greek territory into states for vassal princes: a King of Thessalonica, Prince of Achaea (Greece), Duke of Athens, and Duke of the Archipelago. Boniface, who had met the Byzantine Prince in Germany, became King of Thessalonica. A knight named William of Champlitte became Prince of Achaea; he had to conquer it, but the only resistance was local. A knight named Otto de la Roche became Duke of Athens. You can see where the energy of the Fourth Crusade went; carving up their new territory in ways that had nothing to do with a Crusade.
Thessalonica as a Crusader kingdom didn’t last beyond 1224. The Crusader states of Achaea and Athens lasted a surprising century. They weren’t reconquered by Greeks until about 1308. Through the 1300s, the titles were still passed down, along with some Italian ones like Prince of Taranto, as the minor royalty families merged and kept holding onto hope.
The Greek nobles who had been ruling Constantinople established Empires in exile and kept struggling to retake territory. One family set up in nearby Nicaea, another in Epirus, another in Trebizond. These are known as the “Empires” of those three cities; all three rulers claimed the title.
Until 1261, when the Emperor in Nicaea finally reconquered the city, there were many battles and alliances as the Latins tried to hold on. Their new dynasty didn’t work out, and they never developed a loyal power base. Having to fight on two fronts, against Bulgaria and rebels in Greece, as well as the rival Greek Emperors around Turkey, they were spread too thin.
Surprisingly, some of the Latin rulers in Greece hung on for a long time. For example:
Venice ruled the Ionian Islands until 1864, when they joined the modern state of Greece. It ruled Crete until 1669.
A Latin Count of Salona hung onto power long enough to sell his county to the Knights of the Hospital, who lost it to the Turks in 1410.
The Duchy of Athens was conquered by a group of Catalan mercenaries paid by the Greek Emperor (now back in Constantinople, post 1261), but it became owned by a family in Florence, who ruled it until 1456, when Turks took it.
It’s a mixed history, sometimes with places conquered by Normans in Sicily blurring into the places taken by the Fourth Crusade. If you’re interested, research “Frankokratia” or “Latinokratia.”
Constantinople never really recovered. With so much of its heartland Greek territory lost to Latins, and its eastern border constantly eaten away by Turks, it was ghost of its former strength when besieged by Ottoman Turks in 1453.