British General William Howe, after first gaining control of western Long Island in the Battle of Long Island at the end of August 1776, launched an invasion of Manhattan on September 15. His northward progress was checked the next day in the Battle of Harlem Heights, after which he sought to flank the strong U.S. position on the north of the island. After an abortive landing attempt on October 11, Howe began landing troops in southern Westchester County, New York on October 18, intending to cut off the Continental Army's avenue of retreat. Washington, aware of the danger, withdrew most of his troops north to White Plains. He left a garrison of 1,200 men at Fort Washington under the command of Colonel Robert Magaw; this force was inadequate to fully defend the extensive works. In order to monitor the U.S. garrison in the fort, Howe left Hugh Percy and a small force below Harlem Heights.