 On October 20, General Washington sent Colonel Rufus Putnam out on a reconnaissance mission from his camp at Harlem Heights. Putnam discovered the general placement of the British troop locations and recognized the danger to the army and its supplies. When he reported this to Washington that evening, Washington immediately dispatched Putnam with orders to Lord Stirling, whose troops were furthest north, to immediately march to White Plains. They arrived at White Plains at 9 am on October 21, and were followed by other units of the army as the day progressed. Washington decided to withdraw most of the army to White Plains, leaving a garrison of 1,200 men under Nathanael Greene to defend Fort Washington on Manhattan. General Howe's army advanced slowly, with troops from his center and right moving along the road from New Rochelle to White Plains, while a unit of Loyalists occupied Mamaroneck. The latter was attacked that night by a detachment of Lord Stirling's troops under John Haslet, who took more than thirty prisoners as well as supplies, but suffered several killed and 15 wounded. As a result, Howe moved elements of his right wing to occupy Mamaroneck. On October 22, Howe was reinforced by the landing at New Rochelle of an additional 8,000 troops under the command of Wilhelm von Knyphausen.
