France became a formal American ally after the American victory at Saratoga in 1777.
The Battles of Saratoga took place near Saratoga, New York, on September 19 and October 7, 1777. American forces led by General Horatio Gates defeated the British army commanded by General John Burgoyne, who surrendered on October 17. The outcome demonstrated that the American rebellion could defeat a major British field army.
France had already been secretly supplying the American cause with weapons, ammunition, money, and military assistance. Saratoga persuaded the French government that an open alliance could weaken Britain and improve France’s strategic position. The Treaty of Amity and Commerce and the Treaty of Alliance were signed in February 1778.
French military and naval support became crucial in the later stages of the war. French forces participated in campaigns on land and at sea, including the Yorktown campaign. Spain also fought Britain during the wider conflict, but it did not sign the same formal alliance with the United States in 1778.