The country between the Potomac and the James was cut up by rivers, as the Rappahannock, the Mattapony, and Pamunkey, and part of it was a wilderness. McClellan planned to carry his troops by water to the peninsula between the James and the York and Pamunkey rivers. He would then have a clear road to Richmond, with no great rivers to dispute with the enemy. Johnston would be obliged to leave his camp at Bull Run and march southward to the defense of Richmond. The great objection to the plan was that Johnston might attack Washington instead of going to face McClellan. General Jackson also was in the Shenandoah Valley. He might march down the Valley, cross the Potomac, and seize Washington. So the government kept seventy-five thousand of McClellan's men for the defense of the Federal capital.

On March 8 a queer-looking craft steamed out from Norfolk, Virginia, and attacked the Union fleet at anchor near Fortress Monroe. She destroyed two wooden frigates, the Cumberland and the Congress, and began the destruction of the Minnesota. She then steamed back to Norfolk. This formidable vessel was the old frigate Merrimac. Upon her decks the Confederates had built an iron house. From these iron sides the balls of the Union frigates rolled harmlessly away.