Battles of Saratoga
This success was more than offset by the failure of the expedition to the Hudson, in which General Burgoyne surrendered his army after the Battles of Saratoga, in October. News of Burgoyne's surrender greatly changed the war, because it (and the Battle of Germantown, fought after the British occupied Philadelphia) was a major factor in France's decision to enter the war as an American ally in 1778.