On the evening of March 8, 1862, Secretary of War Edwin Stanton feared a single Confederate ship might steam up the Potomac and shell the White House. The threat was the CSS Virginia, built on the burned hull and surviving steam engines of the frigate USS Merrimack after secessionists took the Gosport Navy Yard in April 1861. Confederate Navy Secretary Stephen Mallory wanted one invincible weapon to break Lincoln’s blockade, and his engineers gave it four inches of iron over 24 inches of iron and pine, sloped at about 36 degrees, ten heavy guns, and a 1,500 pound cast iron ram. That first day the Virginia rammed the USS Cumberland, nearly being dragged down when its ram stuck, then battered the grounded USS Congress with red hot shot until its magazine exploded. The Union lost 250 men and two warships; the Confederacy lost two.

That night John Ericsson’s USS Monitor arrived: a deck 18 inches above the water and a single rotating 120 ton turret with two 11 inch guns. Confederate sailors called it a cheese on a raft. On March 9 the two ironclads fought for four hours at point blank range and could not hurt each other, and each side sailed away believing the other had retreated. Britain and France halted wooden hull construction, monitor mania spread as far as Russia, and navies spent decades fitting rams to their warships. Within the year the Virginia was scuttled by its own crew and the Monitor foundered in a storm off North Carolina, taking 16 men with it.

• The Virginia carried only explosive shells on day two, which shattered harmlessly on iron armor.
• Ericsson ordered the Monitor to use 15 pound powder charges instead of 30, fearing the guns would burst; postwar tests showed full charges would likely have pierced the Virginia.
• A shell hit the Monitor’s pilot house and blinded Captain John L. Worden, handing command to Samuel Dana Greene.
• Commander Franklin Buchanan was wounded on day one, leaving Catesby ap Roger Jones in charge of the Virginia.
• The Virginia’s deep draft kept it from escaping up the James River when Norfolk fell in May 1862.