Mississippi Central Railroad with 40,000 men in an attempt to capture the city of Vicksburg, Mississippi that had begun on November 26. On December 5, Halleck suggested a naval operation down the Mississippi River against Vicksburg, using a portion of Grant's force. A naval operation against Vicksburg had been planned since November by McClernand, who believed he would have full command of such a movement. However, Halleck, who distrusted McClernand, used language in the official orders the latter had received to undercut him: McClernand could only use troops not required by Grant. Grant, who had a rocky relationship with McClernand, tabbed Sherman to lead Halleck's proposed maneuver downriver. After Sherman reached Memphis, to prepare for the movement, Halleck informed Grant that President Abraham Lincoln wanted McClernand to command the naval movement. Grant, in turn, knowing that Sherman was expected to begin the expedition in a matter of days, sent McClernand a letter (intentionally eschewing faster telegraph service) offering him a position under his command. Acting Rear-Admiral David D. Porter was in command of the Union Navy portion of the riverine movement towards Vicksburg. Sherman's men left Memphis on December 20. McClernand was still in Illinois and the movement he had intended to command left without him. The same day that Sherman left, Confederate cavalry raids destroyed Grant's supply lines, and he was forced to retreat. Sherman's operation, which contained over 30,000 men, reached the Vicksburg area on Christmas Eve, and sailed up the Yazoo River. After landing on December 26 and 27, the Union soldiers made a frontal attack against Confederate defenses at Chickasaw Bayou on December 29, but were repulsed with heavy losses. Defeated, Sherman's men re-boarded their ships on January 1, 1863, and withdrew from the battlefield. Preparations When Sherman reached the Mississippi from the Yazoo, he met McClernand, who was upset about being sidestepped in command of the river expedition. After a discussion between the two officers, McClernand took command of the force, naming it the Army of the Mississippi, with Sherman becoming a subordinate commander in the army. Sherman did not approve of McClernand's appointment to command and in a letter to his wife stated that Lincoln "will get his fill before he is done". In late December, Confederate troops had captured a Union supply steamer, Blue Wing, and sent it back to Arkansas Post. Sherman heard of the raid, and believed that more raids from