Which 1777 American victory helped persuade France to become a formal ally of the United States?

The 1777 American victory that helped persuade France to become a formal ally of the United States was the Battles of Saratoga.

The campaign consisted chiefly of the Battle of Freeman’s Farm on September 19 and the Battle of Bemis Heights on October 7. American forces under Horatio Gates and other commanders forced the British army led by General John Burgoyne to surrender at Saratoga, New York, on October 17, 1777.

The victory demonstrated that the Continental forces could defeat a major British army. France had already been secretly supplying the American cause, but news of Burgoyne’s surrender increased French confidence that the rebellion could succeed. France formally recognized the United States and signed the Treaty of Alliance on February 6, 1778.

Saratoga is often called a turning point of the Revolutionary War, but it did not end the conflict. Fighting continued for several years, and French military and naval support became increasingly important. The later American victory at Yorktown in 1781 led to the negotiations that produced the Treaty of Paris.