The day after the British force landed on Long Island, General Washington tried to boost the morale of his soldiers by framing the coming fight as a contest between liberty and slavery.
"The Enemy have now landed on Long Island, and the hour is fast approaching, on which the honor and success of this army, and the safety of our bleeding country depend. Remember officers and soldiers that you are free men, fighting for the blessings of liberty – that slavery will be your portion, and that of your posterity, if you don’t acquit yourselves like men."
General George Washington, General Orders, 23 August 1776The Papers of George Washington, Revolutionary War Series, vol. 6, 13 August 1776 – 20 October 1776, ed. Philander D. Chase and Frank E. Grizzard, Jr. Charlottesville: University Press of Virginia, 1994, pp. 109–111.