|Alexios Comnenos Strategopoulos|
The man credited with returning Constantinople to Byzantine rule was Alexios Comnenos Strategopoulos, who was clearly related to the Comnenos family from which the Byzantine Empire drew several rulers. We don't know when he was born, but records refer to him as an old man in 1258. The earliest mention we have of him is when, in 1252, he was a leader in the army. The blog post linked above mentions that some Byzantine nobles fled the capital to establish their own kingdoms. Alexios lived in one of these, the Empire of Nicaea. His job was to conquer the Despotate of Epirus in northwestern Greece and bring it under Nicaean rule.
Alexios' strategic skills were not stellar, despite his surname, and the campaign failed. Alexios was deprived of his office and imprisoned by Nicaean Emperor Theodore II Laskaris. When Laskaris died in 1258, Alexios was released, and his fortunes started to turn. He went with the army to Macedonia to stop Epirus from taking it, and later was part of the battle that stopped an Achaean-Epiran-Sicilian alliance.
After some other successes and failures, he was sent by Emperor of Nicaea Michael VIII Paleologos with a band of only 800 men to spy on the defenses of the Latins (Venetians and Western Europeans). Michael VIII wanted to take back Constantinople, in the hands of Western Europeans. Fortune smiled upon Alexios: he learned from local farmers that the Venetian fleet and the garrison were gone, conducting a raid against Nicaea. Alexios took a chance that the force would not return, and he led his men—with the help of the locals—via a secret passage into Constantinople, where they took the few guards by surprise and opened the city gate to the Nicaean army of Michael. The next day, 25 July 1261, dawned with Constantinople returning to Greek control.
Learning of the invasion, the Latins living in the city—all of them, including the Latin emperor, Baldwin of Flanders—fled to the harbor to escape. The returning Venetian fleet gave them support during their escape from the oncoming Nicaeans. Michael VIII Paleologos entered the city on 15 August officially and was crowned in Hagia Sophia. Constantinople was finally back under control of the East, and the Byzantine Empire was restored.