Sinope was an important port on the Black Sea coast of Anatolia, at the time held by the Empire of Trebizond, one of the Byzantine Greek successor states formed after the Fourth Crusade.

The Trapezuntine emperor Alexios I (r. 1204–1222) led an army to break the siege, but he was defeated and captured, and the city surrendered on 1 November.

[1][2] According to Ibn Bibi's account, Kaykaus I embarked on the conquest after receiving reports from the frontier that Alexios' troops had been violating Seljuq territory.

According to Selçuk-nâme, 1000 troops led by a commander named Behram cut off the city from the sea, burning ships and killing several Greeks and Western Europeans in the process.

Further, the capture of Sinope provided the Seljuks access to new strategic routes of conquest, one aimed at Constantinople and the other at Crimea and the south Russian steppes.