After Grant's victory at the Battle of Shiloh and at The Battle of Corinth, the Union's prime problem in the west was the capture of the well-defended stronghold of Vicksburg.
After Corinth, Grant marched south down the Mississippi Central Railroad, making a forward base at Holly Springs.
But the enemy in front with 20,000 troops conspired with the enemy in his rear, and not all enemies were Confederate.
Grant's vulnerable supply line went 150 miles. A war Democrat and less-than-competent political general, John A. McClernand, was organizing his own army for an assault on Vicksburg.
McClernand's being a Democrat was of importance. The dominant Republicans needed Democratic support, and Abraham Lincoln gave McClernand the go-ahead, without first telling Grant. Filled with dreams of military and consequent political conquest, the ambitious McClernand began organizing regiments, sending them to Memphis, Tennessee.
Back in Washington, General-in-Chief Henry W. Halleck was likewise nervous about McClernand, giving Grant control of all troops in his own department. McClernand's troops were split into two corps, one under McClernand, one under Sherman. McClernand complained, but to no avail. Grant made off with his troops, one of several maneuvers in a private war within the Union army between Grant and McClernand that would finally come to a head during the The Siege of Vicksburg.
Grant's plan was to have Sherman go downriver while Grant went overland in a two-pronged assault. Grant's telegram to McClernand never made it, as Confederate cavalry commander Nathan Bedford Forrest had cut Grant's communications.
Forrest and Confederate general Earl Van Dorn ended Grant's first effort. Forrest's 2,000 tricky cavalry troops out-fought a number of Union garrisons, while Van Dorn's rode north of Grenada, got behind Grant and destroyed his supply lines, together with his depot at Holly Springs. Van Dorn escaped before Grant could catch him.
Union gloominess was somewhat relieved by their spectacular victory under general William S. Rosecrans over the Confederates and their woefully incompetent general Braxton Bragg at the Battle of Stones River (the Confederates simply called it Murfreesburo, after the city in Tennessee by which at happened). Copperhead anti-Union sentiment somewhat lessened. But Vicksburg stood defiant.
But the primeval Delta apparently sided with the Confederates, trees destroying anything above decks. Confederates felled more trees in the way. A quickly-constructed Confederate fort by Greenwood, Mississippi fired on the Union boats, and the effort collapsed.
Grant's last failure was preceed by seeking a route down the Atchafalaya River, thence to the Red River. They managed a blockade of the Red, but little else.
But the real effort was to dig a canal south of Duckport, aimed at getting lighter boats past Vicksburg. Nothing but the lightest of boats could get through, and this depended upon the height of the river. It came to naught.