Final answer:
The Mongols defeated the Jin Empire by using military tactics, alliances, and exploiting the weaknesses of the Jin dynasty. They gained the support of the Khitan people, laid siege to Zhongdu, and excelled in cavalry warfare.
Step-by-step explanation:
Chinggis Khan and the Mongols defeated the Jin Empire through a combination of military tactics, alliances, and exploiting the weaknesses of the Jin dynasty. Chinggis Khan portrayed his army as a liberating force for the Khitan people, who resented Jin rule, and gained the support of Jin defectors. With their army swelling in numbers, the Mongols laid siege to Zhongdu, the Jin capital, eventually capturing it. The Mongols also excelled in cavalry warfare, which the Jin were ill-prepared for, and the Mongols' naval inexperience hindered their complete conquest of the Song dynasty in southern China.