Hulegu the Ilkhan brought his main force back from Mongolia in 1262. He planned to continue the fight with the Mamluks, trying again to extend his frontier, but now his cousin Berke, Khan in Russia, flew into action. He began attacking Hulegu’s northern territories, creating serious enough invasions that Hulegu had to give up advancing toward Egypt. Their northern border was already “disputed” as we say today; both claimed the Caucasus mountains, both tried to tax its trade. Both were sure they were right.
Berke and Hulegu had another serious conflict as their territories developed competing economic interests. When Batu and his brothers had conquered Crimea, Ukraine, and parts of Hungary previously, they had allowed for the Italian colonies on the Black Sea to keep up a slave trade. In this way, the Golden Horde’s territory was providing most of the slave boys that the Mamluks trained as soldiers. Slavs, Kipchak Turks, and Circassians were much taller than the average Mongol or Arab. If Berke didn’t stop the slave trade, Egypt’s army would keep swelling and then the Ilkhan’s western borders could be pushed back. Mamluks might even take back Baghdad. Of course, this was exactly what Berke wanted.
At first, Berke felt very conflicted about fighting against his cousin Hulegu. It was a prime directive of his grandfather that Mongols must not fight each other, and even more, members of the Golden Family must stay united. But once fighting gets started and trade sanctions begin to take their bite, enmity hardens. And things were just as bad back in Karakorum, where both of Hulegu’s surviving brothers vied to be Great Khan. Berke supported one, Hulegu the other. Kublai tried to get both of them to attend a kurultai in the homeland, but neither would attend.
Both Berke and Hulegu soon died. Hulegu was succeeded by his son Abaka who had already been ruling a city in Iran. The throne in Sarai went to Berke’s nephew, Möngke-Timur, grandson of a different brother. After four years of civil war in Mongolia, Kublai Khan became the Great Khan. He imprisoned his brother Arik Boke and purged his supporters.
But Mongolian civil wars continued: the lineage of Ögedei in Transoxiana was led by Kaidu, Ögedei’s great-grandson. He refused to attend Kublai’s kurultai, which was a tacit vote “against” and a declaration of war. Kublai sent a son of the fourth lineage, Chagatai, to replace him, and it was open war. Eventually the two made a peace treaty and began attacking the Ilkhanate’s Persia. Kaidu never made peace with Kublai, though. Their territories were at war for 30 years, and the Mongol Empire was split. The western lands in Sarai and the Ilkhanate governed themselves separately, while Kublai’s family established the Yuan Dynasty in China.
The different Mongolian branches took on the coloration of the regions they governed. Kublai’s family adopted Chinese culture and Confucianism. The Forbidden City in Beijing started as the inner walled zone where Mongols could still live as Mongols and speak Mongolian without their Chinese subjects watching. Whenever they were in public, they spoke Chinese and acted in an assimilated way. The other lineages gradually adopted Islam, since it was the dominant culture in their regions. There were no more unified attacks on the eastern or western kingdoms by descendants of Genghis Khan, although that culture would create one last ravaging invader, Timur, in the next century.