Mehmed's response came in the summer of 1461. He collected a sizable army at Bursa, and in a surprise move marched on Sinope, whose emir quickly surrendered. Then the Sultan moved south across eastern Anatolia to neutralize Uzun Hasan. Having isolated Trebizond, Mehmed quickly swept down upon it before the inhabitants knew he was coming, and placed it under siege. The city held out for a month before David surrendered on August 15, 1461. With the fall of Trebizond, the last independent remnant of the Byzantine Empire, as well as the Roman Empire from which the Byzantine Empire sprang, was the Empire of Trebizond's offshoot, the Principality of Theodoro. On December 5, 1475 it would also fall to Ottoman rule.