In the fall of 1777, the colonists forced the surrender of 6,000 British troops at the Battle of Saratoga in New York State. It was a huge win for the colonists and a huge loss for the British.
The colonists victory at the Battle of Saratoga turned the tide of the war effort. It was the first major victory for the Continental army.
In May, 1778, impressed by the colonists win at the Battle of Saratoga, and convinced by Benjamin Franklin that it would be in France's best interests to support the colonists, the French joined in the war effort on the side of the patriots.
Britain had help from the Germans (Hessians). The colonists had help from Spain, and now, had help from France. (Later, the Dutch would lend the colonists a hand as well.)