But there were other contributory causes which enabled the Osmanlis so quickly to become masters of the Byzantine Empire and to make themselves a menace to the whole of Europe. Chief among these were the conflicting ambitions of numerous aspirants to the Byzantine throne and the rivalries of the petty chieftains of the Balkan Peninsula and the commercial jealousies of Venice and Genoa. All these purely secular aims made anything like joint action against the followers of the Crescent quite impossible. It was Cantacuzenos, a traitor to his empress, the widow of Andronicus II, who introduced the Osmanlis into Europe. After usurping the Byzantine throne, he gave his daughter, Theodora, in marriage to the emir Orkhan in exchange for six thousand soldiers to aid him in his struggle against his legitimate sovereign. By his infamous betrayal of his empress and country he contributed more than any single factor to the ultimate downfall of the Byzantine Empire. It was the despot Theodore Paleologus who invited the Osmanlis into Greece to support him in his contest with the Greeks and the Franks. It was the Serbian prince Stephen Bukcovitz who formed an alliance with the emir Bayazid to whom he gave his sister as wife and for whom he commanded a contingent in the Ottoman Army--even against his coreligionists. When the Osmanli forces, after their signal defeat by Timur at Angora, were faced with annihilation at the hands of the victorious Tartars, it was the Greeks, Genoese, and Venetians who saved them from destruction by transporting them across the Dardanelles and the Bosphorus to Europe where their relentless pursuers were unable to follow them. But these are only a few instances of the aid which the Ottoman conquerors received from the Christian nations of Europe. “In their conquest of the Balkan peninsula it is remarkable,” declares a recent writer, “that the Osmanlis never fought a battle without the help of allies of the faith and blood of those whom they were putting under the Moslem yoke.”[102] The victories of Bayazid in the region of the Danube were largely due to the coöperation of his Christian vassals. And in his invasion of Hungary he was more beholden to the Wallachians than to his famed corps of Janissaries.[103] Yet more. For, contrary to a common opinion, he received no assistance whatever from Saracens or Persians, for the simple reason that these peoples did not join forces with the Ottomans until a much later date. The Osmanlis did not cross the Taurus until more than a century after they had passed the Balkans and did not become “masters of Asia Minor until long after their inheritance of the Byzantine Empire was regarded in Europe as a _fait accompli_.”[104]