On the ninth of March the immediate anxiety about Washington was relieved. That day came news that the _Monitor_ had checkmated the _Merrimac_ in Hampton Roads and that "Joe" Johnston had withdrawn his forces from the Bull Run position and had retired behind the Rappahannock to Culpeper. On the tenth McClellan began a reconnoitering pursuit of Johnston from Washington. Having found burnt bridges and other signs of decisive retirement, he at last persuaded the reluctant Lincoln to sanction the Peninsula Campaign. On the seventeenth his army began embarking for Fortress Monroe, ten thousand men at a time, that being all the transports could carry. For a week the movement of troops went on successfully; while the Confederates could not make out what was happening along the coast. Everything also seemed quite safe, from the Federal point of view, in the Shenandoah Valley, where General Banks commanded. And both there and along the Potomac the Federals were in apparently overwhelming strength; even though the detectives doing duty as staff officers still kept on doubling the numbers of all the Confederates under arms. Suddenly, on the twenty-third, a fight at Kernstown in the Shenandoah Valley gave a serious shock to the victorious Federals, not only there but all over the semicircle of invasion, from West Virginia round by the Potomac and down to Fortress Monroe. The fighting on both sides was magnificent. Yet Kernstown itself was a very small affair. Little more than ten thousand men had been in action: seven thousand Federals under Shields against half as many Confederates under Stonewall Jackson. The point is that Jackson's attack, though unsuccessful, was very disconcerting elsewhere. From Kernstown the area of disturbance spread like wildfire till the tactical victory of seven thousand Federals had spoilt the strategy of thirty times as many. Shields reported: "I set to work during the night to bring together all the troops within my reach. I sent an express after Williams's division, requesting the rear brigade, about twenty miles distant, to march all night and join me in the morning. I swept the posts in rear of almost all their guards, hurrying them forward by forced marches, to be with me at daylight." Banks, now on his way to Washington, halted in alarm at Harper's Ferry. McClellan, perceiving that Jackson's little force was more than a mere corps of observation, approved Banks and added: "As soon as you are strong enough push Jackson hard and drive him well beyond Strasburg," that is, west of the Massanuttons, where Frémont could close in and finish him. Lincoln had already been thinking of transferring nine thousand men from McClellan to Frémont. Kernstown decided it; so off they went to West Virginia. Still fearing an attack on Washington, Lincoln halted McDowell's army corps, thirty-seven thousand strong, on the march overland to join McClellan on the Peninsula, and kept them stuck fast round Centreville, near Bull Run.