The British army only occupied Philadelphia for one winter, 1777-1778, during which time Washington established a fortified encampment in nearby mountains at Valley Forge. A major attack at Germantown, a suburb of Philadelphia, was launched in October 1777 but failed. Nevertheless, the British realized Philadelphia was untenable and evacuated in the summer of 1778, crossing into New Jersey on a long march to an embarkation point at Sandy Hook. Washington's army stalked the long British column and attacked at Monmouth Court House. The result was a tremendous cannonade and the longest battle of the Revolution. The British fended off any major defeat and proceeded to Sandy Hook.
From a strategic standpoint, the British had the advantage of Delaware Bay and the Delaware River but small American vessels constantly harassed them downriver on the route to Philadelphia, sinking several. Despite superior British naval power, the Americans maintained an advantage.