France became a formal American ally after the Battles of Saratoga in 1777.
The Battles of Saratoga were fought in New York on September 19 and October 7, 1777. The campaign ended with British General John Burgoyne surrendering his army to American General Horatio Gates on October 17. The victory demonstrated that the American rebellion could defeat a major British force.
France had already been secretly supplying the American cause with weapons, ammunition, and other assistance. Saratoga changed French calculations by showing that the United States might actually win independence. In February 1778, France and the United States signed the Treaty of Alliance and the Treaty of Amity and Commerce.
French support brought money, troops, equipment, and naval power into the war. French military and naval assistance later contributed to the British surrender at Yorktown in 1781. Spain also fought Britain during the wider conflict, but it did not sign the same formal alliance with the United States.