Which event led to an open alliance between the French and the United States? a) a request from General Cornwallis b) the Continental Army's winter at Valley Forge c) the Battle of Trenton d) the Patriot victory at Saratoga

It was the Patriot Victory at Saratoga that brought these two nations together in an open alliance. Of course the Americans and French were already somewhat allied because France was also at war with Great Britain during the American Revolution.