In France, enthusiasm for the American cause was high: the French intellectual world was itself in revolt against feudalism and privilege. However, the Crown lent its support to the colonies for geopolitical rather than ideological reasons: the French government had been eager for reprisal against Britain ever since France's defeat in 1763. To further the American cause, Benjamin Franklin was sent to Paris in 1776. His wit, guile and intellect soon made their presence felt in the French capital, and played a major role in winning French assistance.
France began providing aid to the colonies in May 1776, when it sent 14 ships with war supplies to America. In fact, most of the gun powder used by the American armies came from France. After Britain's defeat at Saratoga, France saw an opportunity to seriously weaken its ancient enemy and restore the balance of power that had been upset by the Seven Years' War (the French and Indian War). On February 6, 1778, America and France signed a Treaty of Amity and Commerce, in which France recognized America and offered trade concessions. They also signed a Treaty of Alliance, which stipulated that if France entered the war, neither country would lay down its arms until America won its independence, that neither would conclude peace with Britain without the consent of the other, and that each guaranteed the other's possessions in America. This was the only bilateral defense treaty signed by the United States or its predecessors until 1949.
The Franco-American alliance soon broadened the conflict. In June 1778 British ships fired on French vessels, and the two countries went to war. In 1779 Spain, hoping to reacquire territories taken by Britain in the Seven Years' War, entered the conflict on the side of France, but not as an ally of the Americans. In 1780 Britain declared war on the Dutch, who had continued to trade with the Americans. The combination of these European powers, with France in the lead, was a far greater threat to Britain than the American colonies standing alone.
Source: U.S. Department of State