France provided crucial military support to the American colonists during the American Revolutionary War. France first aided the Patriot cause secretly with weapons, ammunition, money, and supplies, hoping to weaken its longtime rival, Great Britain.
After the American victory at Saratoga in 1777 demonstrated that the rebellion could succeed, France openly allied with the United States. The 1778 Treaty of Alliance brought French troops, warships, loans, and military coordination into the conflict. French forces fought alongside American troops in several theaters, while the French navy challenged Britain at sea.
French assistance was decisive at the Siege of Yorktown in 1781. French soldiers under the Comte de Rochambeau joined Washington’s army, and Admiral de Grasse’s fleet prevented British naval relief. Cornwallis consequently surrendered to the combined American and French force. Spain and the Dutch Republic also opposed Britain or aided the American cause in different ways, but France was the principal foreign military ally and the standard answer to this question.