Berke Khan, the Khan of the Golden Horde was not very fond of his cousin, Helagu Khan, Khan of Ilkhanate primarily because of the latter's actions against the Abbasid Caliphate.
He wrote this to Batu Khan before the fall of Baghdad to garner support from influential Mongol leaders for the Caliph:
"We helped Möngke to enthrone. But he forgot who the enemy is or friend is. Now, he is starving the lands of our friend Caliph. It is abject"
Following the defeat of Abbasids and destruction of Baghdad at hand of Helagu Khan, Berke was consumed with rage and wrote the following letter to the Great Khan, Mongke Khan (Not knowing that the great Khan had already died):
"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."
(see The Mongol Warlords, quoting Rashid al-Din's record of Berke Khan's pronouncement; this quote is also found in The Mamluk-Ilkhanid War)
So it was clear that Berke was already determined to wage war on Ilkhanate to avenge the fall of Baghdad.
It is also stated that Mamluks had developed secret ties with the Golden Horde when they learnt that the Khan was both a Muslim and hated his cousin Helagu for sack of Baghdad.
Also, it is reported that before the decisive battle of Ain Jalut, The Golden Horde provided much needed Cuman-Kipchak reinforcements to Mamluks which proved instrumental in defeating the Ilkhanate hordes.
During the Mongol attack on the Mamluks in the Middle East, most of the Mamluks were Kipchaks, and the Golden Horde's supply of Kipchaks replenished the Mamluk armies and helped them fight off the Mongols.
Halperin, Charles J. 2000. “The Kipchak Connection: The Ilkhans, the Mamluks and Ayn Jalut”. Bulletin of the School of Oriental and African Studies, University of London 63 (2). Cambridge University Press: 229–45.
Afterwards when Helagu returned from Mongolia and decided to avenge the humiliating defeat, He was forced to face the powerful Khan of Golden Horde instead when the latter launched raids into Ilkhanate lands, forcing him to march north instead of west which essentially lead to disintegration of Mongol Empire.
Berke did not want Mongols to fight Mongols as he said:
Mongols are killed by Mongol swords. If we were united, then we would have conquered all of the world.
Johan Elverskog (6 June 2011). Buddhism and Islam on the Silk Road. University of Pennsylvania Press. pp. 186–. ISBN 0-8122-0531-6.
Yet when there was a prospect of a renewed and much larger assault on Mamluks, Berke changed his views and attacked Ilkhanate to save the Mamluks.
Why were the Golden Horde ties to Mamluks stronger than their tie of blood with the Ilkhanate? A simplistic reason would be religion but there has to be something more than that.
I found out some other reasons too for Golden Horde and Ilkhanate rivalry:
- Berke had grievances that Ilkhanate was granted wealthy lands of Persia.
- Berke did not like that Mongke Khan gave Jochi land of Azerbaijan to his brother Helagu.
- Berke was tired of Helagu trying to boss him around with demands of cessation of selling Cuman-Kipchak slaves to Mamluks.
He also took actions against Byzantine Empire through his vassal Bulgaria to force the release of Seljuq Sultan of Rum, Kaykaus II and Egyptian Mamluk envoys. Of course Byzantines were allies of Ilkhanate so naturally Golden Horde must have seen Seljuqs as a buffer between them and Byzantines and did what they could to counter Ilkhanate influence.
The question remains, what did Golden Horde get from this alliance? I can only see the Mamluks benefiting from the alliance with Berke Khan and Golden horde getting nothing out of it. Since bi-lateral relations of states are based on give and take rather than charity, there has to be something the Mamluks had to offer to the Golden Horde.