he Western threat receded when Charles of Anjou, the king of Sicily and main rival of Michael VIII, took part in the crusade against the Muslims of Tunisia undertaken by Louis of France in 1270. This only offered a temporary reprieve, however, since immediately after the end of the crusade, Charles hastened to enforce the terms of the treaty of Viterbo (1267) against Byzantium. To this purpose he proceeded to strengthen the Latins in the Peloponnese, while simultaneously forming an alliance with the Greek rivals of Michael VIII, namely with John, the ruler of Thessaly, and Nikephoros, the despotes of Epiros. In order to be able to confront the powerful anti-Byzantine alliance, Michael VIII took serious diplomatic action. He succeeded in forming an alliance with Hungary against Serbia and with the Tatars of the Golden Horde against the Bulgars. But he could only eliminate Charles I of Anjou with the Pope's help. He therefore once again initiated negotiations of union, which this time led to the union of the two Churches, formalised in 1274 in the city of Lyons. Here, the megas logothetes, George Akropolites, took an oath on behalf of the Emperor, declaring that he recognised the primacy of the Pope and the dogma of the Western Church. The union was ratified in April 1277 by the signing of the confession of faith made by Michael and by his son and joint ruler, Andronikos.

Although the union was considered unacceptable by perhaps the majority of the clergy and the people, and although it caused a deep crisis in the state, it did prove politically beneficial to the Empire. Under the Pope's pressure, Charles I of Anjou was forced to abandon his plans against Byzantium and to accept a truce until 1 May 1276, while in 1275 Venice also renewed its treaty with Byzantium. The diplomacy of Michael VIII had once again achieved its aim.