At nightfall, the British continued their retreat to New York, and Washington moved his army outside the city.[185] Monmouth was Washington's last battle in the North; he valued the safety of his army more than towns with little value to the British.[186]

West Point espionage
Main articles: West Point and Military career of Benedict Arnold, 1777–1779
An 1800 engraving of Washington made after his tenure commanding the Continental Army
Washington became "America's first spymaster" by designing an espionage system against the British.[187] In 1778, Major Benjamin Tallmadge formed the Culper Ring at Washington's direction to covertly collect information about the British in New York.[188] Washington had disregarded incidents of disloyalty by Benedict Arnold, who had distinguished himself in many battles.[189]
In 1780, Arnold began supplying British spymaster John André with sensitive information intended to compromise Washington and capture West Point, a key American defensive position on the Hudson River.[190] Historians[who?]have noted as possible reasons for Arnold's defection to be his anger at losing promotions to junior officers, or repeated slights[clarification needed] from Congress. 