General Washington placed his second-in-command, Lt. Gen. Charles Lee, in charge of organizing the defenses of New York City and its surrounding boroughs. After considering the terrain and possible routes of approach, Lee decided it would be necessary to defend the western portion of Long Island in order to protect the city itself on the southern end of Manhattan Island.
"I wait for some more force to prepare a Post or retrenchd Encampment on long Island opposite to the City for three thoushend Men—this is I think a capital object—for shou’d the Enemy take possession of N. York when Long Island is in our hands—They will find it almost impossible to subsist"
General Charles Lee to General George Washington dated 19 February 1776www.founders.archives.gov