What younger brother of Kublai Khan was defeated in the Toluid Civil War?
The death of Möngke Khan in 1259 CE led to a fight for the Mongol throne that eventually culminated in a civil war between his two younger brothers. Ariq Böke led his army against Kublai Khan in what is known as the Toluid Civil War, as both were members of the Tolui family line. Kublai won the battle, but the legitimacy of the empire was weakened, and the war marked the beginning of the end for the Mongol Empire.
The correct answer is Ariq Böke.