Explanation: When the Mongolian Empire conquered the Kipchaks, a Turkic peoples, many of the captured warriors were sold into slavery, including to the neighboring polity of Egypt.

… when the Mongolian Empire attempted to later invade Egypt, they discovered that many of the Kipchaks they had sold into slavery had been recruited into the (technically) enslaved professional army of the Egyptian Mamluks. Hungry for revenge. And familiar with Mongol tactics.

Oops.

Mongol forces would be repelled, and Egypt would maintain its independence and control of the Levant.