As the siege of Vicksburg progressed, Grant received a steady stream of reinforcements from his own department and departments farther afield. His most immediate source was his own XVI Corps, which was guarding the railroads from Memphis to Corinth. As already mentioned, Lauman's division of this corps arrived on 28 May and was used to extend the siege lines from McClernand's left toward the river. This line was completed when Herron's division from the Department of Missouri came up on 8 June. Kimball's division, newly formed, of XVI Corps reached Grant on 3 June and was used to guard the district north of Vicksburg at Haynes' Bluff. He was reinforced there on 11 June by W.S. Smith's division of the XVI Corps, and by Parke's IX Corps from the Department of Ohio, which arrived on 14 June.
The arrival of all these reinforcements raised Grant's strength from 50,000 at the end of May to 77,000 in mid-June. This enabled him to split his command in two, half to continue the siege of Vicksburg and the other half to keep an eye on Johnston's growing army at Jackson. Grant assigned command of this new force to his trusted lieutenant, William T. Sherman. Sherman's command consisted of Parke's IX Corps and other troops already at or near Haynes' Bluff, a total of 34,000 men. With these he was responsible for a line from Haynes' Bluff to the Vicksburg and Jackson railroad bridge over the Big Black River.
Throughout the siege, Grant had more than just Johnston's force at Jackson to worry about in his rear. Banks was keeping