The battle over Philadelphia was a months-long campaign during the Revolutionary War that ultimately ended with a British evacuation and a very costly American victory. In September 1777, British General Howe led his force of 15,000 men into the Chesapeake to begin the campaign and soon engaged Washington at Brandywine.
The initial skirmishes in the campaign went badly for the Continental Army, and the American forces had to evacuate Philadelphia. Washington's army wintered at Valley Forge, losing 25 percent of its strength to cold and disease. Eventually, the French declaration of war motivated a change in the British command and strategy, and Philadelphia was evacuated by the British army under General Clinton.