The first object of Mahomet's ambition was the conquest of Constantinople, the natural capital of his dominions. As long as it was held by Eastern Christians the Ottoman empire was open to invasion by those of the West. The first threatening act of Mahomet was the construction of a fortress on Constantine's territory, at the narrowest part of the Bosporus, and within five miles of Constantinople. Constantine was too weak to resent the menace with vigor, and Mahomet treated his mild protest with contempt, denying the right of a vassal of the Porte to dispute the Sultan's will. A feeble resistance by some of the Greeks only gave Mahomet pretexts for further aggression, soon followed by his formal declaration of war. Both parties began to prepare for the mortal contest. The siege of Constantinople was to be the great event of the coming year. The Sultan, in order to prevent the Emperor's brothers in the Peloponnesus from sending any succors to the capital, ordered Turakhan, the Pacha of Thessaly, to invade the peninsula. He himself took up his residence at Adrianople, to collect warlike stores and siege artillery. Constantine, on his part, made every preparation in his power for a vigorous defence. He formed large magazines of provisions, collected military stores, and enrolled all the soldiers he could muster among the Greek population of Constantinople. But the inhabitants of that city were either unable or unwilling to furnish recruits in proportion to their numbers. Bred up in peaceful occupation, they probably possessed neither the activity nor the habitual exercise which was required to move with ease under the weight of armor then in use. So few were found disposed to fight for their country that not more than six thousand Greek troops appeared under arms during the whole siege. The numerical weakness of the Greek army rendered it incapable of defending so large a city as Constantinople, even with all the advantage to be derived from strong fortifications. The Emperor was therefore anxious to obtain the assistance of the warlike citizens of the Italian republics, where good officers and experienced troops were then numerous. As he had no money to engage mercenaries, he could only hope to succeed by papal influence. An embassy was sent to Pope Nicholas V, begging immediate aid, and declaring the Emperor's readiness to complete the union of the churches in any way the Pope should direct. Nicholas despatched Cardinal Isidore, the Metropolitan of Kiev, who had joined the Latin Church, as his legate. Isidore had represented the Russian Church at the Council of Florence; but on his return to Russia he was imprisoned as an apostate, and with difficulty escaped to Italy. He was by birth a Greek; and being a man of learning and conciliatory manners, it was expected that he would be favorably received at Constantinople.