General Howe had thus got possession of the city of New York, but the conquest availed him little so long as the American army stood across the island, in the attitude of blockading him. If this campaign was to decide the war, as the ministry hoped, nothing short of the capture or dispersal of Washington's army would suffice. But the problem was now much harder than it had been at Brooklyn. For as the land above Manhattan Island widens rapidly to the north and east, it would not be easy to hem Washington in by sending forces to his rear. As soon as he should find his position imperilled, he would possess the shorter line by which to draw his battalions, together and force an escape, and so the event proved. Still, with Howe's superior force and with his fleet, if he could get up the Hudson to the rear of the American right, and at the same time land troops from the Sound in the rear of the American left, it was possible that Washington might be compelled to surrender. There was nothing to bar Howe's passage up the East river to the Sound; but at the northern extremity of Manhattan Island the ascent of the Hudson was guarded on the east by Fort Washington, under command of Putnam, and on the west by Fort Lee, standing on the summit of the lofty cliffs known as the Palisades, and commanded by Greene. It was still doubtful, however, whether these two strongholds could effectually bar the ascent of so broad a river, and for further security Putnam undertook to place obstructions in the bed of the stream itself. Both the Continental Congress and the State Convention of New York were extremely unwilling that these two fortresses should in any event be given up, for in no case must the Hudson river be abandoned. Putnam and Greene thought that the forts could be held, but by the 9th of October it was proved that they could not bar the passage of the river, for on that day two frigates ran safely between them, and captured some small American craft a short distance above. [Sidenote: Howe moves upon Throg's Neck, but Washington changes base]