The American victory at the Battles of Saratoga in 1777 helped persuade France to enter an open alliance with the United States.
The Saratoga campaign took place in northern New York from June to October 1777. American forces eventually forced British General John Burgoyne to surrender his army at Saratoga on October 17.
The victory demonstrated that the American forces could defeat a major British field army. France had already been secretly supplying the Americans, but Saratoga strengthened the case for a formal alliance and military intervention.
The Franco-American Treaty of Alliance was signed in February 1778. French troops, ships, money, and military support later played an important role in the wider war, including the campaign that led to the British surrender at Yorktown.