Answer:
the colonial victory at the Battle of Saratoga
Step-by-step explanation:
During the early phase of American Revolution, French empire believed that the British empire would easily crushed American colonies. So, the French army did not wanted to spend unnecessary resources by supporting the colonists.
But, the Battle of Saratoga made the French empire changed their decisions.
In that battle, American colonists managed to won the battle despite having lesser military's experience and less developed military's weapons. This prompted France to sign a formal alliance with the colonies during the American Revolution and joined their forces against Britain.