After the sack of Constantinople in 1204, the Byzantine world fragmented into competing successor states. The Empire of Nicaea, established in western Anatolia, claimed legitimacy as the true continuation of the Byzantine Empire and spent most of the thirteenth century working to make that claim real. Under John III Doukas Vatatzes (r. 1221–1254), Nicaea expanded across Thrace and Macedonia, tried and failed to take Constantinople in 1235–1236, and eventually isolated the Latin-held city on its peninsula. By the time Vatatzes died, the Latin Empire controlled Constantinople and a narrow band of surrounding territory. Not much else.

Vatatzes's successor Theodore II Laskaris faced Balkan threats and died in 1258 before he could press the advantage. Power passed to Michael VIII Palaiologos, initially as regent for the infant John IV Laskaris. Michael was ambitious, politically skilled, and militarily experienced. He first secured his western flank: at the Battle of Pelagonia in summer 1259, Nicaea crushed a coalition of Epirus, the Principality of Achaea, and the Kingdom of Sicily. With his chief enemies dead, captured, or in exile, Michael turned toward the city he intended to make his capital.

Michael crossed the Hellespont in January 1260 and advanced toward Constantinople. According to the Byzantine historian George Akropolites, his strategy was not a direct assault — the city walls were too formidable for that — but a conspiracy. A Latin nobleman identified in the sources as 'Asel' (possibly Ansel de Toucy or Ansel de Cahieu) owned a house adjacent to the city walls and had promised to open a gate for the Nicaean troops. Because the plan depended on this insider action, Michael did not bring enough men for a conventional siege. He camped at Galata, across the Golden Horn from the main city, making a show of preparing to attack the Galata fortress while waiting for Asel's signal.

The signal never came. Asel claimed that someone had taken his keys. Michael waited, and the window closed. He negotiated a one-year armistice with the Latin emperor Baldwin II and withdrew. Akropolites, who had a known tendency to minimize Michael's failures, presents this version as the whole story.

Other Byzantine chroniclers — George Pachymeres, Nikephoros Gregoras, and others — describe the 1260 campaign in far grander terms. In their accounts, it was a major military undertaking: a preliminary sweep to capture the forts and settlements controlling the approaches to Constantinople, reaching as far as Selymbria, some 60 km from the city; a direct assault on the fortress of Galata across the Golden Horn; siege engines deployed; attempts made to undermine the walls.

Galata resisted. The inhabitants of the Genoese quarter held their ground, and reinforcements arrived from the main city across the Horn in rowboats — small boats threading back and forth with soldiers and supplies, replenishing the defense. Michael, hearing reports of relief forces approaching, lifted the siege. The two accounts are not irreconcilable: modern historians generally read them as different emphases on the same event, with Akropolites downplaying the scale of the effort to protect Michael's reputation.

The armistice signed in August 1260 gave both sides a year's respite. Michael VIII used the time purposefully. In March 1261, he concluded the Treaty of Nymphaeum with the Republic of Genoa: in exchange for trading rights and privileges, Genoa pledged its warfleet to Nicaea and agreed to a defense pact against Venice, which had been the Latin Empire's primary naval supporter. The treaty was a direct response to the lessons of 1260 — Michael needed naval power to challenge Venetian dominance of the straits.

What actually ended the Latin Empire did not require the Genoese fleet. On 25 July 1261, an advance force sent to scout the city's approaches, commanded by the general Alexios Strategopoulos, discovered that the main Latin garrison had left Constantinople on a naval expedition. The city walls were lightly manned. Strategopoulos found an unguarded entrance, led his troops through in darkness, and took the city. Baldwin II fled by sea. The Latin Empire, which had held Constantinople for fifty-seven years, ended in a few hours. Michael VIII Palaiologos entered the city he had failed to take the year before and was crowned Byzantine emperor in the Hagia Sophia.