France formally allied with the Americans after the 1777 Battles of Saratoga.
The American victory over British forces under General John Burgoyne demonstrated that the rebellion could defeat a major British army. The result persuaded France that supporting the American cause could weaken its longtime rival, Great Britain, while offering France a realistic chance to recover influence and territory.
France and the United States signed the Treaty of Alliance and the Treaty of Amity and Commerce in February 1778. French military aid, money, weapons, troops, and naval power became crucial to the American war effort, especially during the 1781 Yorktown campaign.
France had already provided covert assistance before the alliance. Spain also entered the war against Britain in 1779, but it did not conclude a formal alliance with the United States. That distinction makes France the precise answer to this question.