The major battle that marked a turning point in the American Revolutionary War was the Battle of Saratoga, fought in 1777. This pivotal conflict resulted in a significant victory for the American forces and led to the surrender of General John Burgoyne. The outcome not only boosted the morale of the American troops but also persuaded France to enter the war as an ally of the colonies. This alliance significantly altered the course of the war, providing essential military support and resources that ultimately contributed to American independence.