After waiting five days for supplies, Howe, on the 18th, left Throg's Neck, debarked again on Pell's Point, and on the march northward encountered Glover's brigade well posted behind stone fences. After a hot skirmish Glover slowly fell back, while the enemy advanced to the heights of New Rochelle. Here the British encamped till the 22d, when they were joined by the second division of Hessians under General Knyphausen. This delay gave Washington ample time to strengthen himself at White Plains, where he held a strong and important strategic position commanding the roads leading up the Hudson and to New England.
On the morning of the 28th of October the opposing armies, each about thirteen thousand strong, confronted each other. Washington's intrenchments, partly a double line, occupied the hilly ground within the village of White Plains, the left resting upon a mill-pond and the right on a bend of the Bronx, which protected its flank and rear. Across the Bronx rose Chatterton's Hill, presenting a steep rocky front to the enemy, but it was not fortified.
Howe, believing he was now to fight the decisive battle of the war, moved up in two heavy columns, Clinton commanding the one on the right and De Heister that on the left. They seemed at first as if intending to attack in front; but they soon filed off to the left, extending their line to the front of Chatterton's Hill. Here the main body halted, while a column four thousand strong proceeded to cross the Bronx and storm the hill under cover of the fire of twenty pieces of artillery. General McDougall with fifteen hundred Continentals and militia, and Captain Alexander Hamilton with two pieces of artillery, immediately arrayed themselves on the rocky brow of the hill for its defence. As the main British body, under General Leslie, clambered up the steep acclivity it was met by a withering fire from the infantry and artillery, from which it recoiled and sought shelter. A second assault up the slope met with an equally determined resistance, and for some time the enemy was held in check. Rahl, with two regiments that had forded the Bronx a quarter of a mile below, now appeared on the American right, and drove the militia from their post. This break compelled McDougall, exposed to a heavy fire in front and flank, to retreat across the Bronx to White Plains, though with his six hundred Continentals he maintained an obstinate conflict for an hour, and carried off all his wounded and artillery. The American loss in the engagement was 30 prisoners and 130 killed and wounded, while their opponents' losses were 231.
Howe contemplated an assault, the next morning, upon the American camp, but was deterred by the apparent strength of the lines. These had been built hastily, as General Heath says, of corn-stalks, the tops being turned inwards, and the roots with the adhering earth outwards. The British army, strongly reinforced by the arrival of Lord Percy on the 30th, designed attacking the American works on the following day, but a storm delayed their operations, and gave Washington time to withdraw his forces to the heights of New Castle, where he erected strong defences. In the meanwhile Knyphausen had been ordered to move from New Rochelle to Kingsbridge, where he encamped on November 2d, the Americans retiring to Fort Washington on his approach. Howe in person suddenly left White Plains on the night of the 5th for Dobbs's Ferry, to which his army was already moving. "The design of this manœuvre", wrote Washington on the 6th to the President of Congress, "is a matter of much conjecture and speculation, and cannot be accounted for with any degree of certainty." A council of war which met that day evidently inferred that it threatened a movement across or up the Hudson, for it was unanimously agreed immediately to throw a body of troops into New Jersey, and station three thousand at Peekskill to guard the Highlands. Howe really contemplated a far different move—the capture of Fort Washington.
Why Sir William did not again attack Washington, and why he changed his whole plan, is now well understood to be due to the treason of William Demont, the adjutant of Colonel Magaw, in command of Fort Washington. This man, on the 2d of November, undiscovered, passed into the British camp, and placed in the hands of Lord Percy complete plans of the defences of Mount Washington and a statement of their armament and garrisons. This detailed information was immediately sent, with its author, to Howe, and must have reached him a day or two before his sudden departure from White Plains. The conclusive evidence of this treason is furnished by the culprit himself in his letter,[722] dated London, January 16, 1792, to the Rev. Dr. Peters, of the Church of England, which was first published by Mr. E. F. DeLancey, in the Magazine of American History (Feb., 1877).
Fort Washington, built by Colonel Rufus Putnam soon after the evacuation of Boston, occupied the highest ground at the northern end of Manhattan Island. It was a pentagonal bastioned earthwork without a keep, having a feeble profile and scarcely any ditch. In its vicinity were batteries, redoubts, and intrenched lines. These various field fortifications, of which Fort Washington may be considered the citadel, extended north and south over two and a half miles, and had a circuit of six miles. The three intrenched lines of Harlem Heights, crossing the island, were to the south; Laurel Hill, with Fort George at its northern extremity, lay to the east; upon the River Ridge, near Tubby Hook, was Fort Tryon, and close to Spuyten Duyvel Creek were some slight works known as "Cork Hill Fort;" and across the creek, on Tetard's Hill, was Fort Independence. The main communication with these various works was the old Albany road, crossing Harlem River at Kingsbridge. This road was obstructed by three lines of abatis, extending from Laurel Hill to the River Ridge.
Fort Washington mounted not more than eighteen guns en barbette, of various calibres, from nines to thirty-twos. The garrison of all the various works was less than 3,000 men, mostly Pennsylvanians, who were commanded by Colonel Magaw, an officer of but little military experience. The ground about the fort was well suited for defence, and the works not only protected the upper part of Manhattan Island, but in conjunction with Fort Lee, on the palisades opposite, commanded the Hudson. However, from their too elevated positions and distance from each other, these two works, on the opposite sides of the river, with their feeble armament, proved insufficient, even with a partially constructed barrier of sunken hulks, to prevent the passage of the British vessels-of-war.
As these forts did not close the river, Washington did not deem it expedient to weaken his force, which was necessary to him for field operations, by leaving a large garrison on an island essentially in the hands of the enemy. To the opinion of General Greene, in general command of these works, and in deference to the expressed wishes of Congress to hold them at any cost, Washington yielded his better judgment. His modesty and sense of imperfect knowledge of the science and practice of war led him, as it did on several occasions, to defer too much to others, and though he did not think it "prudent to hazard the men and stores at Mount Washington", he left it discretionary with Greene to give the necessary orders for its evacuation.
Howe, November 15th, demanded the surrender of Fort Washington, stating that, if he were compelled to take it by assault, the garrison would be put to the sword. Magaw replied that to propose such an alternative was unworthy of a British officer, and that, for himself, he should defend the fort to the last extremity.
On the 15th Washington started across the river from Fort Lee, to which he had come, to determine the condition of the garrison at Fort Washington. He says, "I had partly crossed the North River when I met General Putnam and General Greene, who were just returning from thence, and they informed me that the troops were in high spirits and would make a good defence, and, it being late at night, I returned."