The Continental Army forced the redcoats out of Boston in March 1776, but that summer the British captured New York City and its strategic harbor, which they held for the duration of the war. The Royal Navy blockaded ports and captured other cities for brief periods, but they failed to destroy Washington's forces. The Patriots attempted to invade Canada during the winter of 1775–76 without success, but they captured a British army at the Battle of Saratoga in October 1777. France entered the war as an ally of the United States with a large army and navy. The war then moved to the Southern states, where Charles Cornwallis captured an army at Charleston, South Carolina in early 1780, but he failed to enlist enough volunteers from Loyalist civilians to take effective control of the territory. Finally, a combined American and French force captured a second British army at Yorktown in the fall of 1781, effectively ending the war. The Treaty of Paris was signed September 3, 1783, formally ending the conflict and confirming the new nation's complete separation from the British Empire. The United States took possession of nearly all the territory east of the Mississippi River and south of the Great Lakes, with the British retaining control of Canada, and Spain taking Florida.