The ecclesiastical history of the time of the Palaeologi is extremely interesting both from the point of view of the relations between the Greek Eastern church and the papal throne, and from the point of view of the religious movements in the internal life of the Empire. The relations with Rome, which took the form of attempts to achieve union with the Catholic church, were, except the Union of Lyons, closely connected with the ever-growing Turkish danger, for in the opinion of the Byzantine Emperor this danger could be prevented only by the intervention of the pope and the western European sovereigns. The readiness of the pope to favor the proposition of the eastern monarch very often depended upon international conditions in the West.
The Union of Lyons. The popes of the second half of the thirteenth century, in their eastern policy wished no repetition of the Fourth Crusade, which had failed to solve the extremely important problem of the Greek schism, and merely had served to postpone the other important question of a crusade to the Holy Land. Now it seemed desirable to the popes to achieve a peaceful union with the Greeks, which would put an end to the old schism and give grounds to hope for the liberation of Jerusalem.
The recapture of Constantinople by the Greeks in 1261 was a heavy blow to the pope. Papal appeals to save what the Latins had accomplished in the East were sent to many sovereigns. But the papal attitude depended upon affairs in Italy: the popes, for example, did not wish to act with the Hohenstaufen Manfred, whom they hated. Yet when Manfred's power in southern Italy was destroyed by Charles of Anjou, though the latter had been invited by the pope, his aggressive policy against Byzantium found no favor with the papacy. The popes realized that the power of Charles, increased by the conquest of Byzantium, would be hardly less dangerous to the world position of the papacy than the Hohenstaufen sway in Byzantium. It is interesting to note that the first union at Lyons under Michael Palaeologus was achieved not under the pressure of the eastern Turkish danger, but under the menace of the aggressive policy of Charles of Anjou.