British General John Burgoyne's army marches south from Canada to link up with General William Howe's army. Burgoyne had taken Fort Ticonderoga in July. This dealt a tough defeat to the colonial cause. Howe decided to move his troops from New York to the Chesapeake Bay.
The British defeated Washington's army at Brandywine, Pennsylvania on September 11th. The British entered Philadelphia on September 25th. Howe's move left Burgoyne exposed near Saratoga, New York.
On September 19th,the Colonial Army under the command of General Horatio Gates defeated Burgoyne at Freeman's Farm near Saratoga. On October 7th the British were defeated again at Bemis Heights (the Second Battle of Saratoga). Burgoyne surrendered his forces.
This win for the Continentals allowed France to openly enter the war on the side of the Americans.