Final answer:
Cornwallis was defeated at Yorktown because he was trapped on the peninsula and unable to escape. American and French forces laid siege to his army, blocking any escape routes. General Clinton's withdrawal from Yorktown also left Cornwallis vulnerable.
Step-by-step explanation:
Cornwallis was defeated at Yorktown because he was trapped on the peninsula, unable to escape by land or water. A combined force of American and French troops laid siege to Cornwallis's army, effectively blocking any escape routes. Additionally, Cornwallis did not have the support of General Clinton, who had withdrawn from Yorktown, leaving Cornwallis vulnerable.