A few years earlier he had led the Sack of Baghdad; the Islamic Empire was no longer strong enough to retaliate against the size of Hulagu's army. The almost total destruction of Baghdad, however, was to have an unforeseen consequence.
The consequence came from the Golden Horde, a group of settled Mongols who ruled over Russia, Ukraine, Kazakhstan, Moldova, and the Caucasus, established by Batu Khan. Batu Khan was another of Genghis Khan's 43 grandchildren. Batu had died in 1255, and his brother Berke was now in charge of a large territory with a large population. Berke was no lesser figure: not only had he succeeded Batu (there was one ruler between them briefly), but he had been sent by Batu to ensure Möngke's succession as Great Khan, managing the ruling council and ensuring that everything to enthrone Möngke happened properly.
In and of itself this would not seem to be a problem, but Berke had converted to Islam in 1252. The destruction of Baghdad and the knowledge and treasures of the House of Wisdom enraged him, and he vowed revenge, saying "He (Hulagu) has sacked all the cities of the Muslims, and has brought about the death of the Caliph. With the help of God I will call him to account for so much innocent blood." He allied himself with the Mamluks, who were to be Hulagu's next target.
Berke began a series of raids on Hulagu's territories. Hulagu retaliated into the Golden Horde's territory. This was the first serious war between major Mongol areas. Berke showed some reluctance and lamented "Mongols are killed by Mongol swords. If we were united, then we would have conquered all of the world." He could not give in, however: he felt the threat to the Golden Horde was sufficient that he had to declare Islamic jihad against a Mongol leader who was his cousin.
On another front, Hulagu's forces lost an important battle against the Mamluks after the alliance with Frankish forces fell out. He lost control of Palestine and Syria.
Hulagu died 8 February 1264. He was succeeded by his son, Abaqa Khan, who spent the next almost 20 years dealing with civil war with relations because of his father's treatment of Baghdad. Kublai managed to hold the Empire together, mostly, but in the 1290s the competing khanates meant there was no longer total unity, with every part of the empire accepting the authority of the Great Khan.
So what exactly was the Golden Horde, and did it ever change color? It did, and I'll explain more tomorrow.