John, sometimes called the Fifth and sometimes the Seventh of that name, now became sole emperor, and reigned from 1425 to 1448. The two features of his reign which make all incidents in it that are not connected with them of comparative insignificance, are, first, the steady almost unchecked progress of the Turks in south-eastern Europe and in Asia Minor: the encroachment of an overwhelming flood, now apparently receding in one direction, but again sweeping over every obstacle in another, and in reality always steadily advancing and submerging all the Christian populations in the Balkan peninsula: and, second, the efforts of the emperor and those about him to save the remnant of the empire by obtaining the help of Europe. John’s reign was spent in one continuous effort to obtain assistance from the West to save the city and to check the progress of the Turks. Like his predecessors, he addressed himself to successive popes. Perhaps nothing brings more vividly before the reader of European history the power of the occupants of the pontifical chair than the fact that it was taken for granted that from the pope, and the pope alone, that Western aid could be obtained. We have seen that former emperors had looked to the kings of France and England and to other princes, but their aid was sought only on the advice and with the support of Rome. In justice also it must be admitted that no princes recognised so completely as did a long series of popes the expediency and duty of defending Constantinople as the first outwork of the defences of Europe against the forces of Asia, and of aiding its emperors in their efforts to check the Turkish invasion. They were the prime ministers of Western Europe and almost the only persons who regarded the Eastern question as statesmen. Unfortunately, while the popes saw the necessity of preventing the progress of the barbarians, they attached conditions to their offers of help which made them unacceptable and which indeed were impossible: namely, that the Greeks should accept the Union of the Churches, with which Union was associated the supremacy of the pope. A succession of pontiffs during the two hundred years preceding the Moslem conquest of the city worked for Union with marvellous persistency. The same passionate desire for reunion is not less manifest now in the occupant of the chair of St. Peter; but modern efforts are made with this essential difference, that while in the period which concerns us it was believed that reunion could be imposed, every one now recognises that if it is to be brought about, it must be by voluntary and full consent. [Sidenote: Errors in West regarding Orthodox Church.]