George Washington and 12,000
American troops' encampment for the winter of
1777-
1778 during the
American Revolution, near
Philadelphia,
Pennsylvania, now a
national park and
museum.
The troops camped there in December of 1777 because it was traditional not to fight during the winter, and because the British were occupying the city of Philadelphia and it seemed a good idea to keep an eye on them. It was also a protection for the members of the Continental Congress, who had fled to the town of York when the British came to Philadelphia. The winter was notoriously hard, and supply problems meant that it was cold and hungry for many of the people there (Washington included; he didn't live that much better than those he commanded).
Things got better during the spring, though; foreign military officers such as Baron von Steuben came to help the American side, and a new quartermaster, Nathanael Greene, fixed most of the supply problems. When the army finally left the encampment in June of 1778, they were rested, trained, and supplied and went on to perform better in battles than they had before.