Union Major General Ulysses S. Grant began an overland campaign against Vicksburg along the Mississippi Central Railroad in November.

Sherman's movement was defeated in the Battle of Chickasaw Bayou in late December, and Confederate cavalry raids forced Grant to abandon his overland campaign.

McClernand moved downriver, joined Sherman's force, and took command in early January 1863, calling it the Army of the Mississippi.

After moving through a cutoff into the Arkansas River, the expedition began unloading troops downriver from the fort late on January 9.

McClernand's attack was repulsed, but white flags of surrender began to appear over parts of the Confederate line in uncertain circumstances.

Early in the American Civil War, the Union military leadership developed the Anaconda Plan, which was a strategy to defeat the Confederate States of America by controlling its coastline and major rivers.

[2] The strategically important city of Vicksburg, Mississippi, was still in Confederate hands, and it served as a strong defensive position that commanded the river and prevented the Union from separating the two halves of the Confederacy.

[10] Halleck transferred McClernand's newly recruited troops to Memphis, Tennessee, or Helena, Arkansas, as they entered service.

[12] Grant decided to take control of McClernand's accumulated troops, and place them under the command of Major General William T. Sherman, who was to lead them on a riverine expedition down the Mississippi River to Vicksburg.

[14] After Sherman reached Memphis on December 12, Halleck informed Grant that his army was to be split into four corps, and that McClernand was to command the movement downriver.

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) authorizing him to take command of the force moving downriver.

[17] McClernand received communication from Halleck on December 21 confirming that he was intended to command the downriver expedition, but he believed that he was required to remain in Illinois according to his orders.

Telegraphic communications with United States Secretary of War Edwin Stanton, who was not aware of Halleck's machinations against McClernand, cleared up the confusion.

[19] The same day that Sherman left, Confederate cavalry raids destroyed Grant's supply lines, and he was forced to retreat.

[8] Sherman's operation, which contained over 30,000 men, reached the Vicksburg area on Christmas Eve, and sailed up the Yazoo River.

Viewing Arkansas Post as a threat and believing that a victory would restore his men's broken morale,[34] Sherman began planning a movement against the Confederate position.

[35] On December 30, while traveling downriver from Memphis to join the expedition against Vicksburg, McClernand met with Brigadier General Willis Gorman at Helena.

McClernand and Gorman discussed the military situation at Helena, which had been stripped of troops for Sherman's operation, and the Confederate presence at Arkansas Post.

[35] Writing about Sherman's support for the plan, Richard L. Kiper wrote that his motives were "somewhat suspect" and likely based on desiring a victory to restore the damage his reputation had taken from Chickasaw Bayou.

A force of Texans from Garland's brigade was sent forward as skirmishers, and the six cannons of Hart's Arkansas Battery were positioned on the right flank of the line of rifle pits, near the river.

[50] Morgan's transports halted at a landing on the opposite side of the river, 9 miles (14 km) downriver from Notrebe's, to offload Colonel Daniel Lindsey's brigade.

[56] McClernand ordered Sherman to move to the northwest and try to get around the Confederate fort and into its rear, but he later decided that the river road was also a feasible approach route.

Colonel Giles Smith's brigade of Stuart's division pushed forward after the Confederates, led by two Missouri regiments as skirmishers.

[71] The rifle pits from the fort to Post Bayou were defended by the rest of the 19th Arkansas Infantry on the right, then Garland's brigade, and Deshler's on the left.

[73] Churchill spoke to his men, and told them "gentlemen, the fight will commence in a very short time, and we must win it or die in the ditches".

[75][a] In response to the information from the scouting party, McClernand order Steele's division moved from its camp in the Union rear to form on the right of Stuart's line.

[88] Porter then sent Rattler, Glide, and the ram USS Monarch upriver past the fort to cut off the Confederates' path of retreat.

The historian Timothy B. Smith suggests that it is possible that fire from Porter's ships and Lindsey's artillery striking the rear of the Confederate lines prompted the raising of the surrender flags.

[120] When Grant learned of McClernand's expedition to Arkansas Post, he became angry and wrote to Halleck on January 11, describing the movement as a "wild-goose chase".

[125] On January 14, in a meeting with Sherman and Porter, McClernand proposed moving up the Arkansas River and striking against the Confederate force at Brownsville.