France provided decisive naval support at Yorktown in 1781.
French naval forces under the Comte de Grasse defeated a British fleet at the Battle of the Chesapeake in September 1781. That victory allowed the French fleet to control access to the Chesapeake Bay and prevented the British army at Yorktown from receiving effective relief by sea.
France also supplied troops and military resources on land. French soldiers under the Comte de Rochambeau joined Washington’s Continental Army, while French artillery and engineering expertise strengthened the siege. The combined Franco-American force then surrounded Charles Cornwallis’s British army.
The alliance had begun formally with the Treaty of Alliance of 1778, after the American victory at Saratoga encouraged France to support the rebellion against Britain. French assistance was costly for France, but it was central to the Yorktown campaign. The British surrender helped lead to the peace negotiations that ended the war.