The engineers reported late on Saturday, the 20th, a practicable crossing of the stream at Sudley Ford, accessible by a detour of five or six miles around a bend of Bull Run turning sharply from the west. McDowell determined to send Hunter's and Heintzelman's divisions to make this flank movement over a route which took them north, then west, and brought them upon the enemy's left, as they crossed Bull Run at Sudley Ford and moved due south by the Sudley Road toward Manassas. Meanwhile Tyler was ordered to proceed from Centreville to the Stone Bridge at Bull Run, there to feign attack until he heard Hunter and Heintzelman engaged, when he would cross and join their attack on the Confederate left, or push on to Gainesville, west of Bull Run, and head off Johnston, who McDowell was certain was coming from Winchester, with or without "Patterson on his heels," as General Scott had promised. But during McDowell's enforced two days of inactivity at Centreville there had been portentous happenings within the Confederate lines. Johnston had already left Winchester on the 18th; one detachment of his army had joined Beauregard on the morning of the 20th; Johnston in person arrived at noon with a second detachment, and the remainder of his force arrived on the 21st in time to take part in the battle, the brunt of which was borne by Johnston's army, which McDowell had hoped not to meet at all! Johnston, as the ranking officer, assumed command, and he and Beauregard turned their attention to defending themselves against the attack now initiated by McDowell.