After the succession was settled and his brother Kublai Khan was established as Great Khan,
Hulagu returned to his lands by 1262.
When he massed his armies to attack the Mamluks and avenge the defeat at Ayn Jalut, however, he was instead drawn into civil war with Batu Khan's brother Berke.
Berke Khan, a Muslim convert and the grandson of Genghis Khan, had promised retribution in his rage after Hulagu's sack of Baghdad and allied himself with the Mamluks.
He initiated a large series of raids on Hulagu's territories, led by Nogai Khan.
Hulagu suffered a severe defeat in an attempted invasion north of the Caucasus in 1263.
 This was the first open war between Mongols and signaled the end of the unified empire.
In retaliation for his failure, Hulagu killed Berke's ortogh, and Berke did the same in return.

Even while Berke was Muslim, out of Mongol brotherhood he at first resisted the idea of fighting Hulagu.
He said, Mongols are killed by Mongol swords.
If we were united, then we would have conquered all of the world.
But the economic situation of the Golden Horde due to the actions of the Ilkhanate led him to declare jihad because the Ilkhanids were hogging the wealth of North Iran and because of the Ilkhanate's demands for the Golden Horde not to sell slaves to the Mamluks.
After the succession was settled and his brother Kublai Khan was established as Great Khan,
Hulagu returned to his lands by 1262. When he massed his armies to attack the Mamluks and avenge the defeat at Ayn Jalut, however, he was instead drawn into civil war with Batu Khan's brother Berke. Berke Khan, a Muslim convert and the grandson of Genghis Khan, had promised retribution in his rage after Hulagu's sack of Baghdad and allied himself with the Mamluks. He initiated a large series of raids on Hulagu's territories, led by Nogai Khan. Hulagu suffered a severe defeat in an attempted invasion north of the Caucasus in 1263.  This was the first open war between Mongols and signaled the end of the unified empire. In retaliation for his failure, Hulagu killed Berke's ortogh, and Berke did the same in return.
Even while Berke was Muslim, out of Mongol brotherhood he at first resisted the idea of fighting Hulagu. He said, Mongols are killed by Mongol swords. If we were united, then we would have conquered all of the world. But the economic situation of the Golden Horde due to the actions of the Ilkhanate led him to declare jihad because the Ilkhanids were hogging the wealth of North Iran and because of the Ilkhanate's demands for the Golden Horde not to sell slaves to the Mamluks.
Hulagu Khan