It is easy to see that each of these rare and isolated points afforded a position by the fortification of which the passage of an enemy could be disputed, and the control of the stream maintained, as long as it remained in the hands of the defenders. They were all, except Columbus and Hickman, in territory which, by the act of secession, had become hostile to the Government of the United States; and they all, not excepting even the two last-named, were seized and fortified by the Confederates. It was against this chain of defences that the Union forces were sent forth from either end of the line; and fighting their way, step by step, and post by post, those from the north and those from the south met at length around the defences of Vicksburg. From the time of that meeting the narratives blend until the fall of the fortress; but, prior to that time, it is necessary to tell the story of each separately. The northern expeditions were the first in the field, and to them this chapter is devoted. The importance of controlling the Mississippi was felt from the first by the United States Government. This importance was not only strategic; it was impossible that the already powerful and fast-growing Northwestern States should see without grave dissatisfaction the outlet of their great highway pass into the hands of a foreign power. Even before the war the necessity to those States of controlling the river was an argument against the possibility of disunion, at least on a line crossing it. From the military point of view, however, not only did the Mississippi divide the Confederacy, but the numerous streams directly or indirectly tributary to it, piercing the country in every direction, afforded a ready means of transport for troops and their supplies in a country of great extent, but otherwise ill-provided with means of carriage. From this consideration it was but a step to see the necessity of an inland navy for operating on and keeping open those waters. The necessity being recognized, the construction of the required fleet was at the first entrusted to the War Department, the naval officers assigned for that duty reporting to the military officer commanding in the West. The fleet, or flotilla, while under this arrangement, really constituted a division of the army, and its commanding officer was liable to interference, not only at the hands of the commander-in-chief, but of subordinate officers of higher rank than himself.