Who was important in the American victory at Yorktown

US General George Washington.

Explanation:

The Battle of Yorktown (1781) represents one of the key episodes of the American Revolutionary War. The American troops were led by George Washington and had the help of the French Count Rochambeau. The operation's culmination was the last stage of the battle when American troops won the battle in the open field. That detail increased the morale of American soldiers. The last battle resulted in the capture of about 7,000 British soldiers, which encouraged the British to start their diplomacy and organize negotiations.

to extend a country’s power and influence