France became a formal American ally after the Battles of Saratoga in 1777.
The Battles of Saratoga were fought near Saratoga, New York, on September 19 and October 7, 1777. American forces commanded by Horatio Gates defeated the British army led by John Burgoyne, whose troops surrendered after the second battle. The victory demonstrated that the American rebellion had a realistic chance of defeating Britain.
News of Burgoyne’s surrender persuaded King Louis XVI’s government to negotiate openly with the United States. France signed the Treaty of Amity and Commerce and the Treaty of Alliance with the United States on February 6, 1778. French support included soldiers, naval forces, weapons, supplies, and financial assistance.
French intervention globalized the war and forced Britain to divide its military resources among several theaters. French naval support also played a decisive role at Yorktown in 1781. Although Spain and the Dutch Republic later opposed Britain, France was the power that entered the formal alliance directly after Saratoga.