You don your snorkeling gear and scour the bottom of the lake for your next clue: "The lost territory of this river flows color." You know that early on the Mongols took all territory north of which Chinese river?

After launching an attack on a Chinese northwest border-state and gaining the allegiance of his king, Khan continued in his campaign against north China, resulting in the 1215 fall of Beijing. Everything north of the Yellow River was then Mongol territory.

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