Selected by the Continental Congress to serve as general-in-chief, his first actions were to blockade Boston. Key to the success in Boston was the capture of Dorchester Heights, allowing cannon fire against the British and forcing the withdrawal of Howe. After failing to defend New York, he retreated toward Pennsylvania, extending British supply lines and allowing a successful counterattack on Hessian mercenaries at Trenton. Following victory at Princeton, this general retired to winter quarters at Morristown. Sending his best forces north to deal with Burgoyne's attack in spring 1777, he kept Howe engaged in the mid-Atlantic. Autumn setbacks at Brandywine and Germantown led to a demoralized winter camp at Valley Forge, countered by the work of Lafayette, Steuben, and others. After a costly draw with Sir Henry Clinton's forces at Monmouth, Washington sent Greene south to replace Gates, and worked with the French general Jean Baptiste Rochambeau to plan the Yorktown campaign. The success of this campaign led to Cornwallis' surrender on October 19, 1781.