France became America’s formal ally after the American victory in the Battles of Saratoga.
The Battles of Saratoga were fought in New York in September and October 1777. American forces under General Horatio Gates defeated and ultimately compelled the surrender of British General John Burgoyne’s army. The victory demonstrated that the American rebellion could defeat a major British field force.
France had already been secretly supplying the American cause, but Saratoga changed the diplomatic calculation. In 1778, France signed the Treaty of Amity and Commerce and the Treaty of Alliance with the United States. French military, naval, financial, and material support then became an important part of the wider war.
The alliance internationalized the conflict. French naval operations and troops helped put pressure on Britain in several theaters, including the Yorktown campaign of 1781. Saratoga is therefore often called a turning point, although the war continued for years afterward.