Which 1777 victory persuaded France to openly ally with the American revolutionaries?

The American victory at the Battle of Saratoga persuaded France to openly ally with the revolutionaries in 1778.

The Saratoga campaign took place in northern New York in 1777. American forces first checked British troops at Freeman’s Farm on September 19 and then defeated them at Bemis Heights on October 7. British General John Burgoyne surrendered his army at Saratoga on October 17.

The victory showed France that the American rebellion had a realistic chance of success. France signed the Treaty of Amity and Commerce and the Treaty of Alliance with the United States in February 1778, providing military, financial, and naval support.

Saratoga was not the final victory of the Revolution; Yorktown came later in 1781. Its major international importance was turning American battlefield success into a formal Franco-American alliance.