France joined the American Revolutionary War after the American victory at Saratoga.
The Battles of Saratoga took place near Saratoga, New York, on September 19 and October 7, 1777. The campaign ended when British General John Burgoyne surrendered his army to American General Horatio Gates on October 17. The defeat showed that the American rebellion could defeat a major British force.
France had already been providing secret assistance, including supplies and weapons. Saratoga persuaded the French government that the American cause had a realistic chance of success. France formally entered the war against Britain in 1778 through the Treaty of Alliance with the United States.
French aid transformed the conflict. France supplied troops, money, weapons, and naval power, including the fleet that helped trap Cornwallis at Yorktown in 1781. Spain also fought Britain during the wider war, but it did not sign the same alliance with the United States or enter the conflict immediately after Saratoga.