Sea power was destined to play a great part in the conflict. As soon as the Washington Government realized that it was going to be a serious and prolonged war, not an affair of a few weeks, a general plan of operations was devised, of which the essential feature was the isolation of the Southern Confederacy. When the crisis came in 1861 the United States had done little to open up and occupy the vast territories between the Rocky Mountains and the Mississippi Valley. The population of the States was chiefly to be found between the Mississippi and the Atlantic, and in that region lay the states of the Confederacy. They were mainly agricultural communities, with hardly any factories. For arms, munitions of war, and supplies of many kinds they would have to depend on importation from beyond their frontiers. It was therefore decided that while the United States armies operated on the northern or land frontier of the Confederacy, its sea frontiers on the Atlantic and the Gulf of Mexico should be closely blockaded, and its river frontier, the line of the Mississippi, should be seized and held by a mixed naval and military force. For these last operations troops on the banks and gunboats on the river had to combine. It was said at the time, that on the Mississippi army and navy were like the two blades of a pair of shears, useless apart, but very effective when working together. Strange to say, it was not the industrial North, but the agricultural South, that put the first ironclad into commission as a weapon against the coast blockade. When the Secessionist forces seized the Navy Yard at Norfolk, in Virginia, a fine steam frigate, the "Merrimac" (built in 1855), was under repair there. The guard of the dockyard set her on fire before surrendering, but the flames were extinguished, and the "Merrimac," with her upper works badly damaged, was in possession of the Southerners. A Northern squadron of frigates and gunboats, steam and sailing ships, anchored in Hampton Roads, the landlocked sheet of water into which runs not only the Elizabeth River, which gives access to Norfolk, but also the James River, the waterway to Richmond, then the Confederate capital. The northern shores of Hampton Roads were held by Federal troops, the southern by the Confederates. Presently spies brought to Washington the news that the "Rebels" were preparing a terrible new kind of warship at Norfolk to destroy the squadron in Hampton Roads and raise the blockade.