The Fall of Outremer
The Mongols
All through the 1250s Outremer was protected from its follies by truces that were more or less respected. In addition, everyone saw what they thought was a new hope rising in the East: the Mongols.
This was the age of the greatest of the Khans, including Jenghiz and Kublai, and of the creation of the Golden Horde. The Mongol court was religiously eclectic and many religions could be found there. The Mongols weren't so much tolerant as they were open, being willing to have all sorts of religions in their empire. They only demanded prompt payment of taxes, political subservience, and men and supplies for their armies.
Christianity had worked its way eastward as well as westward from its birthplace, and the most important Christian sect in central Asia was the Nestorian. They could be found everywhere, and even the Khan Hulagu had a Nestorian wife. As they encountered Muslim territories, the Mongols tended to favor Christians and to suppress Muslims, not least because the Muslims were their chief opponents in the region.
As news of Christian influence among the Mongols filtered westward, it became colored by the hopes and fears of the Greeks and Latins. The Mongols came to be seen as rescuers sent by God to destroy Islam. The legends about a fabulous Christian prince of the East, Prester John, got tangled up in this as well.
In short, the Mongols were seen as allies. This was a profound misunderstanding of them. They would seek alliances, but only on terms that clearly set the Mongols as the superior party and the ally as the junior. But for a couple of decades, this was not at all clear in the West. So, even as the Mameluks destroyed castle after castle in Outremer, there was hope that the Mongols would soon set all to rights.