France became the United States’ formal ally during the American Revolution in 1778.
The alliance followed the American victory at Saratoga in 1777, which convinced French leaders that the rebels had a realistic chance against Britain. On February 6, 1778, the two countries signed the Treaty of Amity and Commerce and the Treaty of Alliance.
French assistance included loans, weapons, military advisers, soldiers, and naval forces. France’s navy was especially important in the campaign that trapped British forces at Yorktown in 1781.
France joined the war against Britain, but it was not acting simply as a neutral helper. French leaders also wanted to weaken their long-standing rival and recover influence lost during earlier conflicts. Spain entered the wider war against Britain in 1779, but it did not sign the same formal alliance with the United States.