France became the United States’ formal ally through the 1778 Treaty of Alliance.
The treaty was signed in Paris on February 6, 1778, alongside a Treaty of Amity and Commerce. It transformed French support for the American rebels into a formal alliance against Great Britain. France recognized American independence and agreed to provide military assistance.
French aid proved crucial to the American war effort. France supplied money, weapons, soldiers, and naval forces, and its fleet helped prevent British relief during the Yorktown campaign. French military cooperation under the Comte de Rochambeau and naval command under the Comte de Grasse were especially important in 1781.
Spain and the Dutch Republic also fought Britain during the wider conflict, but neither was the country named in the 1778 Treaty of Alliance with the United States. The Franco-American alliance remained in force until the Treaty of Paris ended the war in 1783.