Answer:
During the next decades an uneasy coexistence prevailed between the Mongols in northern China and the Song state in the south. The Mongols resumed their advance in 1250 under the grand khan Möngke and his brother Kublai Khan grandsons of Genghis Khan. Their armies outflanked the main Song defenses on the Yangtze River and penetrated deeply into southwestern China, conquered the independent Dai (Tai) state of Nanzhao in what is now Yunnan, and even reached present-day northern Vietnam. Möngke died in 1259.
Step-by-step explanation:
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