State of the Byzantine Empire
Constantinople had been an imperial capital since its consecration in 330 under Roman emperor Constantine the Great. In the following eleven centuries, the city had been besieged many times but was captured only once before: the Sack of Constantinople during the Fourth Crusade in 1204
The crusaders established an unstable Latin state in and around Constantinople while the remainder of the Byzantine Empire splintered into a number of successor states, notably Nicaea, Epirus and Trebizond. They fought as allies against the Latin establishments, but also fought among themselves for the Byzantine throne.
The Nicaeans eventually reconquered Constantinople from the Latins in 1261, reestablishing the Byzantine Empire under the Palaiologos dynasty. Thereafter, there was little peace for the much-weakened empire as it fended off successive attacks by the Latins, Serbs, Bulgarians and Ottoman Turks.