The 1777 American Revolutionary War victory that helped bring France into an alliance with the United States was the Battle of Saratoga.
The Battles of Saratoga took place in New York in September and October 1777. American forces commanded by Horatio Gates and Benedict Arnold defeated and forced the surrender of British General John Burgoyne’s army. The campaign ended a major British attempt to split New England from the other colonies.
News of the victory strengthened the American diplomatic position in France. French leaders already opposed Britain and had secretly supplied the Patriot cause, but Saratoga demonstrated that the Americans could defeat a major British field army. On February 6, 1778, France signed treaties of alliance and commerce with the United States.
Saratoga is often called a turning point of the Revolutionary War, though military historians also emphasize later campaigns and the importance of French aid. French troops, naval forces, weapons, money, and supplies became essential to the American war effort. French participation helped make the eventual victory at Yorktown possible.