France became a formal American ally through the 1778 Treaty of Alliance.
The treaty was signed in Paris on February 6, 1778, by representatives of France and the United States. It followed the American victory at Saratoga and recognized the United States as an independent power for diplomatic purposes. France had already been providing covert assistance, including weapons and supplies.
The agreement promised French military support and committed both parties not to make a separate peace with Britain. France entered the war openly, turning the colonial conflict into a wider international war involving major European powers and naval forces across the Atlantic.
French aid included troops, ships, money, and supplies. French naval operations were especially important during the Yorktown campaign in 1781, when the French fleet helped prevent British reinforcements from reaching General Cornwallis. The alliance did not itself end the war; peace came through the Treaty of Paris signed in 1783.