That Gregory realized clearly the peril to Christendom in the advance of Murad’s armies is shown in two remarkable letters written to Louis of Hungary in May and November 1372. His words were prophetic. He urged Louis to resist the Osmanlis before they advanced farther into Europe. They had already entered Serbia. He trembled to think what would happen if they pushed through Albania and secured a port on the Adriatic. Unless Louis entered without delay into an alliance with his Christian neighbours, how could he protect his own kingdom and all Christendom from the Mohammedan peril.[301] Seconding this warning to the King of Hungary, the Pope commanded the Hungarian and Slavic archbishops to preach the crusade in Hungary, Poland, and the Dalmatian cities. Everywhere special boxes were placed in the churches for collecting funds. A tithe was levied on the monasteries and abbeys of Hungary and Dalmatia. Louis, with five of his most powerful nobles, took the cross, and swore to the Pope that he would put an army in the field within a year.[302] Louis asked Venice for triremes, but when the Venetians found that he intended them to be a donation for ‘the common cause’, they found that they could not build them.[303] Padua declined an invitation to guarantee the cost of construction. The Hungarians did not fulfil their promises. In fact, there is no evidence that they made any effort to acquit themselves of their oath. When John Palaeologos made a last desperate appeal to the Pope, before he entered into his third and final compact with Murad, Gregory, in receiving the imperial envoy, burst into tears, and promised that he would save Constantinople, if only the Byzantine emperor would cause his people to renounce their heresies and return to the Roman Church. In 1375, he wrote once more to Louis to inform him that Constantinople was in danger of capture from Murad.[304] Letters in the same year to Edward of England pictured the Ottoman advance and the peril of Christendom, urged a general war against the Osmanlis, and asked for a subsidy to provide galleys ‘to prevent the crossing into Europe of more Turks, because Constantinople is in imminent danger’.[305] The letters of Gregory XI to the Christian princes prove conclusively that the full import of Murad’s early successes was understood by the Pope and was impressed upon both secular and ecclesiastical authorities throughout Europe.