Perhaps it is slightly premature to say that the Mississippi was opened from the hour of the surrender of Vicksburg. Port Hudson held out, but its fall was a corollary of that of the more important city. It had stoutly resisted several attacks, but, realizing the hopelessness of his situation, the Confederate commander surrendered on the 9th of July, and the opening of the Mississippi was fully completed. ROSECRANS' CAMPAIGN. The reader will recall that the battle of Murfreesboro' took place at the very beginning of the year. Rosecrans, the Union commander, never repeated the brilliant skill he had shown in fighting Bragg on Stone River. He seemed to think that that repulse of the enemy was sufficient to last a good while, for he remained idle throughout the several months that followed. There were a number of brisk skirmishes and fights, but none was of importance. When June arrived without anything of account having been accomplished, the government suggested to Rosecrans that it was time he took steps to drive Bragg into Georgia and thus secure Eastern Tennessee, where the sentiment was strongly Union. Rosecrans hesitated, but upon receiving a stronger intimation that he ought to be up and doing, he began a series of movements, in the latter part of June, which caused Bragg to withdraw to Chattanooga, where he intrenched himself. Burnside then advanced from Ohio into Eastern Tennessee, but was so delayed that Bragg was heavily reinforced from Virginia. To protect his communications, he fell back, however, upon the approach of the Federal army, which occupied Chattanooga. Unaware of the increased strength of the enemy, Rosecrans divided his army into three columns, separated by wide spaces of mountains, and marched in loose order against his foe, observing which Bragg determined to overwhelm each of the columns in detail. The first demonstration was against General George H. Thomas, who commanded the Federal left, and was encamped at the foot of Lookout Mountain. That splendid officer eluded the enemy launched against him, and effected a junction with the other two corps. At the same time the centre of the three columns was attacked, but the assault was repulsed, and the reunited Union army on the 18th of September stood on the western bank of the Chickamauga, which stream was well named, for the Indian word means "the river of death." The position was twelve miles from Chattanooga, and it was a perilous one, for, as has been stated, Bragg had been heavily reinforced, and Longstreet with a powerful column of veterans from Lee's Army of Northern Virginia was approaching. He, therefore, decided to make an attempt to recover Chattanooga. BATTLE OF CHICKAMAUGA.