Which Revolutionary War campaign victory helped persuade France to become an official American ally?

The Battles of Saratoga helped persuade France to become an official American ally during the Revolutionary War.

The two major battles took place near Saratoga, New York, on September 19 and October 7, 1777. American forces under General Horatio Gates defeated the British army commanded by General John Burgoyne, who surrendered on October 17. The American victory demonstrated that the Continental Army could defeat a major British force.

News of the surrender encouraged France to enter the war openly on the American side. The Franco-American alliance was formally signed in 1778, bringing French troops, naval power, weapons, supplies, and financial assistance to the American cause. French intervention transformed the conflict into a global war and contributed significantly to the eventual British defeat.

Saratoga is sometimes described as a single battle, but the name properly refers to two principal engagements in the campaign. Yorktown was the later decisive siege that led to the British surrender in 1781.