Several attempts at a Franco-Mongol alliance against the Islamic caliphates, their common enemy, were made by various leaders among the Frankish Crusaders and the Mongol Empire in the 13th century. Such an alliance might have seemed an obvious choice: the Mongols were already sympathetic to Christianity, given the presence of many influential Nestorian Christians in the Mongol court. The Franks (Western Europeans and those in the Crusader States of the Levant were open to the idea of support from the East, in part owing to the long-running legend of the mythical Prester John, an Eastern king in an Eastern kingdom who many believed would one day come to the assistance of the Crusaders in the Holy Land. The Franks and Mongols also shared a common enemy in the Muslims. However, despite many messages, gifts, and emissaries over the course of several decades, the often-proposed alliance never came to fruition.
Contact between Europeans and Mongols began around 1220, with occasional messages from the papacy and European monarchs to Mongol leaders such as the Great Khan, and subsequently to the Ilkhans in Mongol-conquered Persia. Communications tended to follow a recurring pattern: the Europeans asked the Mongols to convert to Western Christianity, while the Mongols responded with demands for submission and tribute. The Mongols had already conquered many Christian and Muslim nations in their advance across Asia, and after destroying the Nizaris of Alamut (the Assassins, TN) and the Muslim Abbasid and Ayyubid dynasties, for the next few generations fought the remaining Islamic power in the region, the Egyptian Mamluks. Hethum I, king of the Christian nation of Cilician Armenia, had submitted to the Mongols in 1247, and strongly encouraged other monarchs to engage in a Christian–Mongol alliance, but was only able to persuade his son-in-law, Prince Bohemond VI of the Crusader state of Antioch, who submitted in 1260. Other Christian leaders such as the Crusaders of Acre were more mistrustful of the Mongols, perceiving them as the most significant threat in the region. The Barons of Acre therefore engaged in an unusual passive alliance with the Muslim Mamluks, allowing Egyptian forces to advance unopposed through Crusader territory to engage and defeat the Mongols at the pivotal Battle of Ain Jalut in 1260.
European attitudes began to change in the mid-1260s, from perceiving the Mongols as enemies to be feared, to potential allies against the Muslims. The Mongols sought to capitalize on this, promising a re-conquered Jerusalem to the Europeans in return for cooperation. Attempts to cement an alliance continued through negotiations with many leaders of the Mongol Ilkhanate in Persia, from its founder Hulagu through his descendants Abaqa, Arghun, Ghazan, and Öljaitü, but without success. The Mongols invaded Syria several times between 1281 and 1312, sometimes in attempts at joint operations with the Franks, but the considerable logistical difficulties involved meant that forces would arrive months apart, never able to coordinate activities in any effective way. The Mongol Empire eventually dissolved into civil war, and the Egyptian Mamluks successfully recaptured all of Palestine and Syria from the Crusaders.
After the fall of Acre in 1291, the remaining Crusaders retreated to the island of Cyprus. They made a final attempt to establish a bridgehead at the small island of Ruad off the coast of Tortosa, again in an attempt to coordinate military action with the Mongols, but the plan failed, and the Muslims responded by besieging the island. With the Fall of Ruad in 1302, the Crusaders lost their last foothold in the Holy Land.(...)
There was not much support among the general populace in Europe for a
Mongol alliance. Writers in Europe were creating "recovery" literature
with their ideas about how best to recover the Holy Land, but few
mentioned the Mongols as a genuine possibility. In 1306, when Pope Clement V asked the leaders of the military orders, Jacques de Molay and Fulk de Villaret,
to present their proposals for how the Crusades should proceed, neither
of them factored in any kind of a Mongol alliance. A few later
proposals talked briefly about the Mongols as being a force that could
invade Syria and keep the Mamluks distracted, but not as a force that
could be counted on for cooperation.
This blog quotes parts of an article on Wikipedia on the subject. Go to the full article for many more details as well as ample references. The illustration shows the 289 letter of Arghun to Philip IV of France, in the Mongolian script, with detail of the introduction. The letter was conveyed to the French king by Buscarel of Gisolfe. Souce Wikipedia, Public Domain. The translated text reads: "Under the power of the Eternal Heaven. Under the majesty of the Khan (Kublai Khan). Arghun our word. To the Rey da France (King of France). Last year you sent your ambassadors led by Mar Bar Sawma telling us: "if the soldiers of the Il-Khan ride in the direction of Misir (Egypt) we ourselves will ride from here and join you", which words we have approved and said (in reply) "praying to Tengri (Heaven) we will ride on the last month of winter on the year of the Tiger and descend on Dimisq (Damascus) on the 15th of the first month of spring." Now, if, being true to your words, you send your soldiers at the appointed time and, worshipping Heaven, we conquer those citizens (of Damascus together), we will give you Orislim (Jerusalem). How can it be appropriate if you were to start amassing your soldiers later than the appointed time and appointment? What would be the use of regretting afterwards? Also, if, adding any additional messages, you let your ambassadors fly (to us) on wings, sending us luxuries, falcons, whatever precious articles and beasts there are from the land of the Franks, the Power of Tengri and the Majesty of the Khan only knows how we will treat you favorably. With these words we have send Muskeril (Buscarello) the Khorchi. Our writing was written while we were at Khondlon on the sixth khuuchid of the first month of summer on the year of the Ox." Red seal imprints with the same six seal script Chinese characters: 輔國安民之寶 Fǔguó ānmín zhībǎo ("Precious seal of the upholder of the State and the purveyor of peace to the People").