Byzantine–Mongol alliance

A Byzantine-Mongol Alliance occurred during the end of the 13th and the beginning of the 14th century between the Byzantine Empire and the Mongol Empire. Byzantium attempted to maintain friendly relations with both the Golden Horde and the Ilkhanate realms, and was caught in the middle of growing conflict between the two. The alliance involved numerous exchanges of presents, military collaboration and marital links, but dissolved in the middle of the 14th century.
In the spring of 1242, John III Doukas Vatatzes, Emperor of Nicaea initiated a campaign against the Thessalonian Empire, and besieged its capital, Thessalonica. Soon after, the Mongols invaded the Sultanate of Iconium, causing Vatatzes to end the campaign early. Soon after the Battle of Köse Dağ in 1243, the Empire of Trebizond surrendered to the Mongol Empire while the court of Nicaea put its fortresses in order. In the early 1250s, the Latin emperor of Constantinople Baldwin II sent an embassy to Mongolia in the person of the knight Baudoin de Hainaut, who, following his return, met in Constantinople with the departing William of Rubruck. William of Rubruck also noted that he met an envoy of John III Doukas Vatatzes, Emperor of Nicaea, at the court of Möngke Khan around 1253.
Following the Mongol partition of the Sultanate of Rum between the pro-Mongol Kilij Arslan IV in the east and the pro-Nicaean Kaykaus II in the west, the Nicaean emperor Theodore Doukas Laskaris engaged in active diplomacy with the Ilkhanate, receiving a Mongol embassy in 1257. Through Laskaris' shrewd deception, the embassy was convinced that Nicaea was a large and powerful state with a formidable army and covered entirely by mountains, thus making it exceedingly difficult for the Mongols to subjugate. The Mongol ambassadors were therefore content with Nicaea remaining independent in exchange for Rum being recognized as a Mongol protectorate. The embassy also lead to negotiations for a marriage alliance between the two rulers, however Laskaris died in 1258 before the alliance could be finalized.
Emperor Michael VIII Palaiologos, after re-establishing Byzantine Imperial rule, established an alliance with the Mongols, who themselves were highly favourable to Christianity, as a minority of them were Nestorian Christians.
He signed a treaty in 1266 with the Mongol Khan of the Kipchak (the Golden Horde), and he married two of his daughters (conceived through a mistress, a Diplovatatzina) to Mongol kings: Euphrosyne Palaiologina, who married Nogai Khan of the Golden Horde, and Maria Palaiologina, who married Abaqa Khan of Ilkhanid Persia.
According to a 1267 letter by Pope Clement IV from Viterbo, Abaqa had agreed to combine forces with his father-in-law Michael VIII to help the Latins in the Holy Land, in preparation for the Eighth Crusade (the second of Louis IX):
Mamluk envoys were traveling through Constantinople to the Golden Horde in the summer of 1263. However, the leader of the Ilkhans would have been angered if Michael allowed the envoys to pass, leading Michael to send them back to Egypt. In response, the Golden Horde army invaded Nicaean territory, freeing the former Seljuk Sultan Kaykaus II, who had been imprisoned on Nicaean territoriy since rebelling and fleeing from the Mongols. The conflict ended after the Mamluks convinced the Golden Horde to withdraw from Byzantine territory, and the Byzantines to allow the Mamluk envoys to travel to the Golden Horde.
According to Egyptian sources, Michael agreed to send fabrics to the Mongol Khan in Russia. When Michael realized the importance of the Mongols and became an ally of Noghai, he used his help to defend himself against Bulgaria when it tried to attack the Byzantine Empire in 1273 and 1279. Probably in 1276, Maria Palaiologina Kantakouzene, Michael's niece, planned an attack on Byzantium. She withdrew after Michael convinced the Mongols to raid Bulgaria. Later, in 1278, the Byzantines and the Mongols jointly besieged and captured Maria in Tirnovo. A group of 4,000 Mongol soldiers were dispatched to Constantinople in 1282, just before the death of Michael, to fight against the despot of Thessaly.
After 1295, Andronikos II offered Ghazan a marital alliance, in exchange for Mongol help to fight against the Turcomans at the Oriental frontier of the Byzantine Empire. Ghazan accepted the offer and promised to stop the incursions. The death of Ghazan in 1304 was mourned by the Byzantines.
This alliance would continue under Ghazan's successor, Oljeitu. In 1305, Ilkhan Oljeitu promised Andronikos II 40,000 men, and in 1308 dispatched 30,000 men to Bithynia to recover many Byzantine towns. Andronicus II gave daughters in marriage to Toqta, as well as his successor Uzbek (1312–1341), but relations turned sour at the end of Andronikos's reign and the Mongols mounted raids on Thrace between 1320 and 1324, until the Byzantine port of Vicina Macaria was occupied by the Mongols in the late 1330's.
Mongol control of western and central Anatolia was unstable, which allowed Turkmen groups to raid and cause damage to many frontier villages in the Byzantine Empire. These raids would eventually lead to the complete collapse of Byzantine Anatolia under Andronikos II. The Byzantines asked the Ilkhans for help in 1302-1303 and again in 1304-1305, but despite promises of help, it did not stop the Turkmen advances.
Under Andronikos III relations seem to have turned even more conflictual. In 1341, the Mongols planned to attack Constantinople, and Andronikos III had to send an embassy to stop the attack.