The Greco-Bulgarian alliance. Next, John Asen, irritated by his failure to obtain the regency at Constantinople, took the lead in an alliance of the Orthodox rulers of the East, composed of Asen himself, John Vatatzes of Nicaea, and Manuel of Thessalonica. This new union was directed against the Latins. One cannot help seeing in the formation of this alliance a dangerous step for the interests of the Bulgars in the Balkan peninsula. Thereby, as V. G. Vasilievsky correctly stated, Asen, the soul of the coalition, contributed to the friendly understanding between Manuel of Thessalonica and the Emperor of Nicaea, between the European and Asiatic Greeks, and opened the way to the Nicene master to extend his influence in the former Empire of Thessalonica and even in Asen's own dominions. The restoration of the orthodox Eastern Empire was partly decided by this rapprochement. An important result of this alliance for the internal history of Bulgaria was the recognition there of the autocephalous Bulgarian patriarchate, which was established with the consent of the Nicene and other eastern patriarchs.
The capital of the Latin Empire, surrounded on all sides by enemies, was again in a very dangerous position, which was well realized by contemporaries. The aim of the offensive alliance against the Latins was the complete destruction of Latin domination, the expulsion of the Latins from Constantinople, and the division of their possessions between the allies. The troops of Asen and Vatatzes besieged Constantinople in 1235, by land and sea, but were compelled to withdraw without definite results. In his letter appealing to the West for help for the Emperor of Constantinople, the alarmed Pope Gregory IX declared that Vatatzes and Asen, schismatics, who had recently concluded an alliance of impiety, had invaded with numerous Greek troops the land of our dearest son in Christ, the Emperor of Constantinople. Driven to despair, Baldwin II, the last Latin Emperor, left Constantinople and traveled through western Europe, begging rulers for help for the Empire in men and money.
For the time Constantinople was saved. One cause for the stopping of the advance of the Orthodox alliance was the gradual withdrawal of John Asen himself, who realized that in the Empire of Nicaea he had a more dangerous enemy than in the dying and weakened Latin Empire. Accordingly the king of Bulgaria changed his policy and came out as a defender of the Latin Emperor. Simultaneously with this change of political combinations, Asen took steps towards reconciliation with the papal throne, announcing his faithfulness to the Catholic church and asking the pope to send a legate for negotiations. Thus the short Greco-Bulgarian alliance of the fourth decade of the thirteenth century came to its end.
A History of the Byzantine Empire - Table of Contents
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Reference address : https://www.ellopos.net/elpenor/vasilief/john-asen-ii.asp?pg=3