France formally allied with the Americans after the 1777 Battles of Saratoga.
The Battles of Saratoga took place near Stillwater, New York, on September 19 and October 7, 1777. The second battle ended with British general John Burgoyne's surrender on October 17. The victory demonstrated that the Continental forces could defeat a major British army in the field.
France had already been secretly supplying the American cause with weapons, ammunition, and other assistance. Saratoga convinced the French government that the rebellion had a realistic chance of success. In February 1778, France and the United States signed the Treaty of Alliance and the Treaty of Amity and Commerce.
French support transformed the conflict into an international war. French troops, loans, supplies, and naval forces helped the Americans continue the struggle, while France opened additional military pressures against Britain. Spain and the Dutch Republic later fought Britain, but France was the first foreign power to enter a formal alliance with the United States during the Revolution.