Now the Southern armies, flushed with victory, took the aggressive and invaded the North. They swept across Maryland and entered Pennsylvania, no effective opposition being offered. Hooker and his army started after them, but in the last week of June Hooker was removed from command, and General George Gordon Meade was put in his place. That wise and capable leader hurried the Union army northward, and on July 1st confronted Lee at Gettysburg. There, on July 1st, 2d, and 3d, was fought the greatest battle of the war, and one of the most important in human history. It cannot be described in detail here, but it resulted in the complete discomfiture of the Confederates, who retreated with all possible haste back to Virginia, and never sought to invade the North again. General Meade followed them, but was unable to overtake and capture them. During the remainder of that year Meade made two attempts upon Richmond, but without important results. Thus matters stood in Virginia at the beginning of 1864, when a new factor appeared upon the scene, before dwelling upon which some events elsewhere must be recounted. Attacks had been made, up to this time, upon the Confederates along the coast by several expeditions. General T. W. Sherman and Commodore Du Pont had occupied Beaufort in November, 1861. Early in 1862 General Burnside had taken Roanoke Island and Newberne. In the West, beyond the Mississippi, there had been much fighting, especially in Arkansas, and the National arms had been generally successful. On the water, also, the National fleets were supreme. At no time had the Confederates a fleet able to hold its own at sea. They had a number of fast cruisers, fitted out in England, which roamed the ocean as freebooters, preying upon American commerce. The most notable of these was the “Alabama,” which was finally destroyed off Cherbourg, France, by the “Kearsarge,” in June, 1864. They had also a number of powerful rams and ironclad gunboats, for coast and harbor defense. Most famous of these was the “Merrimack,” which, in Hampton Roads, destroyed the great frigates “Congress” and “Cumberland,” and bade fair to deal likewise with the whole Union fleet. Opportunely, the little ironclad “Monitor,” just built by John Ericsson, appeared upon the scene, gave battle, and vanquished the monster “Merrimack.” This was one of the epoch-making naval battles of the world. It not only saved the whole Union fleet, and perhaps many Northern seaport cities from destruction. At a single stroke it revolutionized naval architecture and naval warfare. The great wooden frigates were instantly made things of the past; thenceforth the typical war-ship was a heavily armored iron and steel machine, carrying only a few guns in revolving turrets, or in heavy iron casemates.