Final answer:
The foreign contribution that led to the American victory at Yorktown was the French naval blockade, which prevented the British navy from providing reinforcements or supplies to General Cornwallis. This allowed the combined American and French armies to lay siege to Cornwallis's army and led to his surrender.
Step-by-step explanation:
The foreign contribution that led to the American victory at Yorktown was the French naval blockade. This blockade prevented the British navy from providing reinforcements or supplies to General Cornwallis, who was trapped in Yorktown.
The French naval forces, commanded by Admiral de Grasse, played a crucial role in cutting off Cornwallis's escape route by sailing into Chesapeake Bay. This allowed the combined American and French armies, led by General Washington, to lay siege to Cornwallis's army, ultimately leading to his surrender.