According to the Office of the Historian, Benjamin Franklin persuaded France to aid the colonists in their rebellion against the British. While Franklin was popular and admired in France, however, it was ultimately the British surrender at the Battle of Saratoga in December 1777 that encouraged the French to join.
On Feb. 6, 1778, representatives from France and the American colonies signed the Treaty of Alliance, and France was officially in the war. This treaty forbade either America or France from making a separate peace with Great Britain, and it allowed the further entrance of European powers into the war. Spain joined the war on June 21, 1779.