Wearied out by constant struggle, Cantacuzenus states that he wished to abdicate and retire to a monastery, and that his wife approved of his design. His writings show that he felt great interest in the discussion of theological questions. The part which he himself took in several religious controversies, the anxiety that he underwent to have the excommunication against him annulled, first by the Patriarch John and afterwards, ‘for greater safety,’ by John’s successor,[58] Isidore, his negotiations with the pope for Union, and many other circumstances, show that the withdrawal to a monastery was a not unnatural development of his life. While he was making preparations to carry his design into execution, news came of the progress of Stephen of Serbia, which forced him to postpone it. Salonica, ‘one of the eyes of the empire,’ was in danger of surrendering to Stephen. The partisans of the Palaeologi among the population of that city were numerous. The neighbouring country was, however, under the power of the great Serbian, and unless Stephen were checked without delay the city would be given over to him. The old emperor sent word to his followers to remain steadfast, promising that he would come to their relief. In order to do so, he took a step which is sometimes incorrectly treated as the first important introduction of the Turks into Europe.[59] He induced his son-in-law, Orchan, to send a body of twenty thousand cavalry, under his son Suliman, across the Dardanelles to march against Stephen. The emperor left the capital as soon as he had heard that the Turks had crossed the straits to co-operate with them, and took his co-emperor John, who was obnoxious to the Turks, with him. For some reason which is not clear, the Othman or Ottoman Turks withdrew after they had crossed the Maritza, but the two emperors with another body of Turks went to Salonica and put an end to any design to surrender it. This was in 1349. The history of the empire during the next six years is a medley of incidents, due to the hostility between the two emperors. John refused to address his elder colleague as emperor, and even proposed to join Stephen of Serbia, whose power in the Balkan peninsula was now greater than that of any other ruler. The Bulgarian king, appealed to by Cantacuzenus to enter into alliance against Stephen, refused his co-operation, and shortly after joined the Venetians to attack the empire. [Sidenote: Genoese and Venetians.]