Answer:
There was a civil war between Kublai and his younger brother.
Explanation:
Enthronement and civil war. Kublai received a message from his wife that his younger brother Ariq Böke had been raising troops, so he returned north to the Mongolian plains. ... This kurultai proclaimed Kublai Great Khan, on April 15, 1260, despite Ariq Böke's apparently legal claim to become khan.
Kublai Khan as Yuan Dynasty Emperor
His efforts paid off, with much of the Song imperial family surrendering to Kublai in 1276, but the war continued for another three years. In 1279, Kublai became the first Mongol to rule all of China when he conquered the last of the Song loyalists
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