Washington’s third movement into Westchester County (July, 1780) had a curious connection with Arnold’s treachery. Very few people in America knew that a French army was about to appear at Newport. But Benedict Arnold knew, because Washington, who admired and trusted Arnold, told him about Rochambeau’s imminent arrival. Arnold immediately transmitted this exceedingly valuable intelligence to the British.
Arnold’s intelligence was received with the utmost seriousness at British headquarters in New York by Commander-in-Chief General Sir Henry Clinton, who decided to strike at the French soon after their landing, when Rochambeau’s weary soldiers would be most vulnerable to attack. Sir Henry took a risk, however, for an attack on Rochambeau required the deployment of thousands of troops far from Manhattan. His regiments had already been depleted because of Britain’s new southern campaign, which had involved the removal of large numbers of men from New York to the Carolinas. By mustering additional troops for an attack on Rochambeau, Clinton was leaving a skeleton force to defend Manhattan.
Washington’s movement into Westchester County was triggered by news of Rochambeau’s arrival and by information simultaneously received from his spies in Manhattan, who told him that Clinton was marshaling thousands of troops and sending them east.
Realizing that Rochambeau must be Clinton’s target, Washington quickly moved his army from its camp sites in northern New Jersey, crossed the Hudson at King’s Ferry (from Stony Point to Verplanck), and positioned the Continental troops in the Peekskill area, ready to march south to threaten Manhattan. He hoped through that maneuver to alarm Clinton, lure him back to the undermanned Manhattan garrison, and thereby protect the French.
Washington’s maneuver achieved its intended purpose. Clinton could not accept the risk to New York posed by the Continental Army, and he abandoned his plans to pounce on Rochambeau. As the British regiments returned to New York, Washington withdrew the American Army from Westchester. His third movement into the County had been brief and successful.