But both John and Gregory lost heart. Neither was able to fulfil the compact made in Rome. Gregory could not unite Christendom to relieve the Byzantines. John could not persuade the Byzantines to renounce, as he had done, the ‘Greek heresies’. So, as we have seen, he became Murad’s vassal.[306] The Pope, involved in the quarrel of Emperor Charles IV and the Duke of Bavaria with the Marquis of Brandenburg, and anxious over the outcome, for the papacy, of the continual unrest in the Italian cities, returned from Avignon to Rome in 1378. He died a few months later.[307] The struggle arising from the election of Gregory’s successor gave birth to the ‘Great Schism’. This left Murad a free hand in subjugating the Balkan peninsula. VIII The sources of information for the movements from the outside for the relief of the Balkan Christians, and for the religious and political quarrels of the Byzantines, are so numerous and so detailed that one is embarrassed by too much material. Many interesting facts cannot even be mentioned. But when we come to the beginning of the Ottoman conquest in Europe under Murad and Bayezid, we find ourselves in the midst of what an eminent Slavic historian has called ‘the most obscure and difficult period of South-Slavic history’.[308] The chroniclers, whether they be Slavic, Rumanian, or Ottoman, are so contradictory and so lacking in explicit statement that we cannot speak with certainty of the sequence of events. The Byzantine chroniclers, verbose to the point of weariness in detailing petty and trifling quarrels and happenings, are almost silent concerning the momentous events that marked the ruin of their empire. It is difficult to unravel the twisted skeins, and find a thread to carry the story of the conquest from 1366 to 1389. When it is impossible to choose between contradictory records, the geography of the field of action, with which one can gain a first-hand knowledge, must be the final factor in determining the sequence of conquest between the adoption of Adrianople by Murad as his capital and the downfall of the Serbians at Kossova.