The French were the greatest ally of the American colonists. France was intrigued at the prospect of helping the Americans, as it would be a way to get back at Britain after France's loss in the French and Indian War.
Marquis de Lafayette was a French aristocrat who served on Washington's staff. France joined the war officially in 1777 after the American victory at Saratoga. France threatened British possessions all over the world and diverted valuable assets away from the colonial war. French troops also served on American soil. The greatest French contribution was the French navy, keeping the British navy at bay during the siege of Yorktown, where Washington would capture Cornwallis's army. While this 1781 battle is largely considered the end of the war, the war would actually drag on another two years. Washington hoped that the French would assist him in retaking New York City, but the French commander moved to harass British possessions in the Caribbean, thus proving that even at this late date, the sugar islands of the Caribbean were still considered the prize of the New World.
France also gave the American colonial government significant financial help at a time when the French treasury was suffering from grave inefficiencies of its own. This was key when the Continental Congress could only ask the states for money. France's joining the war on the American side led the Spanish and the Dutch to do the same, as France was still the major superpower on the European continent.