Updated: Nov 2
The Trans-Mississippi region saw a multi-pronged approach by the Union to invade Texas. Neither the Union's Camden Campaign, Bank's Red River flotilla, nor the dispersed skirmishes in Indian Territory led to a successful take-over of Texas. While its neighbors suffered in stopping the Union armies from entering, North Texas itself left the Civil War relatively unscathed.
By July 1863, the Confederacy was not doing too well. It had been cut in two by Grant's forces after the Siege of Vicksburg, and Lee's armies were retreating from gains made into Union controlled states. The Union claimed the capitals of Arkansas and Louisiana, but the local governments in the western areas of the states refused to acknowledge the occupation. Because Indian Territory was not a state, it had never officially seceded, but sectionalism within the tribal nations contributed to chaos and anarchy. The lone exception was Texas.
Texas proved itself to be a juggernaut in the war. In May of 1861, Texas Confederate William C. Young invaded Indian Territory and took over several western forts. By late 1861, Texas troops, led by John R. Baylor, took over towns in New Mexico Territory. In 1862, John Sibley's troops took possession of federal forts in New Mexico and Arizona Territories, but lack of supplies and men forced a retreat. In 1862 and 1863, Texas successfully repelled the Union navy from Galveston and Sabine Pass. Until 1865, Texan Samuel Bell Maxey commandeered the Indian Territory. Throughout the war, Texas merchant ships exported cotton and imported munitions from Cuba, much to the chagrin of the Union.
The Red River region, with its cotton, access to federal territories, and viable shipping ports, proved to be the one area that the Union army had failed to penetrate. According to chroniclers of the period, Abraham Lincoln believed taking Texas, which during the Civil War was the Confederacy's biggest supplier of food as well as the largest "holder" of slaves (meaning, slavers hid their enslaved people in Texas to avoid confiscation) would bring a swift end to the conflict. Besides, the Union feared that Mexico, under the government of Emperor Maximillian of Austria, might use this period of chaos to invade the United States through Texas -- or even be recruited to do so with confederate Texas's help.
With the Union having taken control of Little Rock and New Orleans, the next step was to invade Texas... and since the Union couldn't do it by sea, it saw its opportunity in breaching the Red River Valley. By targeting the Red River, the Union could of course approach Texas, but it also could cut off the pockets of rebellion throughout the western region of Arkansas and Louisiana. The Confederacy surmised the strategic importance of the Red River Valley, too. Edmund Kirby Smith of the Trans-Mississippi Department explained to Jefferson Davis that "... the only true line of operations by which the enemy can penetrate the department is the valley of the Red River, rich in supplies, with steamboat navigation..."
In the Summer of 1863 to the Spring of 1864, three Union campaigns descended almost simultaneously upon the Red River Valley, in the hopes of eventually invading Texas. None of the campaigns were successful for the Union.
The Confederate victories at Mansfield and Pleasant Hill were decisive. The Confederate troops effectively stopped the take-over of Shreveport and an impending invasion of Texas. In fact, the Red River Campaign is still used in naval academies to demonstrate military blunder. Still, the victories did not matter. By May of 1864, William Tecumseh Sherman (who had been a university president in Alexandria before the war!) had started his Georgia and Virginia Campaigns, which eventually led to the capture of Richmond and to the surrender of Robert E. Lee in April, 1865. Though it took Texas until May of 1865 to concede that it lost the Civil War, and until June of 1865 to concede that it had lost its slave labor system, the wins of the Trans-Mississippi Department did not effect the outcome of the Civil War.