After the 1777 Battles of Saratoga, France entered the American Revolutionary War as an American ally.
The Battles of Saratoga were fought near Saratoga, New York, on September 19 and October 7, 1777. The second battle ended with the surrender of British General John Burgoyne’s army to American forces commanded by General Horatio Gates. The victory demonstrated that the Continental Army could defeat a major British field force.
France had already been secretly providing supplies and weapons to the American revolutionaries, but Saratoga encouraged a formal alliance. The Treaty of Amity and Commerce and the Treaty of Alliance were signed in February 1778. French military and naval support later played a major role in the campaign that ended at Yorktown.
Spain also fought Britain during the wider conflict, but it did not sign the same formal alliance with the United States. France is therefore the specific answer to the question about the ally prompted by Saratoga’s victory.