By the beginning of September 1864, events in the eastern United States, especially the Confederate defeat in the Atlanta campaign, gave incumbent president Abraham Lincoln, who supported continuing the war, an edge in the 1864 United States presidential election over George B. McClellan, who favored ending the war. At this point, the Confederacy had very little chance of winning the war. Meanwhile, in the Trans-Mississippi Theater, the Confederates had defeated Union attackers during the Red River campaign in Louisiana, which took place from March through May. As events east of the Mississippi River turned against the Confederates, General Edmund Kirby Smith, Confederate commander of the Trans-Mississippi Department, was ordered to transfer the infantry under his command to the fighting in the Eastern and Western Theaters. This proved to be impossible, as the Union Navy controlled the Mississippi River, preventing a large-scale crossing. Despite having limited resources for an offensive, Smith decided that an attack designed to divert Union troops from the principal theaters of combat would have an effect equivalent to the proposed transfer of troops, by decreasing the Confederates' numerical disparity east of the Mississippi. Price and the new Confederate Governor of Missouri, Thomas Caute Reynolds, suggested that an invasion of Missouri would be an effective offensive; Smith approved the plan and appointed Price to command the offensive. Price expected that the offensive would create a popular uprising against Union control of Missouri, divert Union troops away from principal theaters of combat (many of the Union troops previously defending Missouri had been transferred out of the state, leaving the Missouri State Militia as the state's primary defensive force), and aid McClellan's chance of defeating Lincoln in the election.
