Which battle in 1777 helped persuade France to openly support the American cause?

The Battle of Saratoga helped persuade France to openly support the American cause in 1777.

The Battles of Saratoga took place in New York during September and October 1777. American forces led by Horatio Gates and other commanders defeated a British army commanded by John Burgoyne, which surrendered at Saratoga on October 17.

The victory demonstrated that the American forces could defeat a major British army. France had already provided covert assistance, but Saratoga convinced French leaders that the rebellion had a realistic chance of success. France then signed the Treaty of Alliance with the United States in February 1778, bringing open military support into the war.

Saratoga was not the final American victory; that distinction belongs to the later Yorktown campaign. It was nevertheless a major turning point because French soldiers, supplies, money, and naval power substantially strengthened the American war effort. The alliance also widened the conflict beyond the original North American theater.