In the Eastern theatre of war lies Washington, the capital of the Union, a place of great importance to the North for obvious reasons, and especially because if it fell European powers would be likely to recognise the Confederacy. It lies, on the Potomac, right upon the frontier; and could be menaced also in the rear, for the broad and fertile trough between the mountain chains formed by the valley of the Shenandoah River, which flows northward to join the Potomac at a point north-west of Washington, was in Confederate hands and formed a sort of sally-port by which a force from Richmond could get almost behind Washington. A hundred miles south of Washington lay Richmond, which shortly became the capital of the Confederates, instead of Montgomery in Alabama. As a brand-new capital it mattered little to the Confederates, though at the very end of the war it became their last remaining stronghold. The intervening country, which was in Southern hands, was extraordinarily difficult. The reader may notice on the map the rivers with broad estuaries which are its most marked features, and with the names of which we shall become familiar. The rivers themselves were obstacles to an invading Northern army; their estuaries, on the other hand, soon afforded it safe communication by sea. In the Western theatre of war we must remember first the enormous length of frontier in proportion to the population on either side. This necessarily made the progress of Northern invasion slow, and its proper direction hard to determine, for diversions could be created by a counter-invasion elsewhere along the frontier or a stroke at the invaders' communications. The principal feature of the whole region is the great waterways, on which the same advantages which gave the sea to the North gave it also an immense superiority in the river warfare of flotillas of gunboats. When the North with its gunboats could get control of the Mississippi the South would be deprived of a considerable part of its territory and resources, and cut off from its last means of trading with Europe (save for the relief afforded by blockade-runners) by being cut off from Mexico and its ports. Further, when the North could control the tributaries of the Mississippi, especially the Cumberland and the Tennessee which flow into the great river through the Ohio, it would cut deep into the internal communications of the South. Against this menace the South could only contend by erecting powerful fortresses on the rivers, and the capture of some of them was the great object of the earlier Northern operations.