BUT by 1864 the military situation had changed. There really was no further need to "occupy" any new territory west of the Mississippi River by the Union Army. The Confederates in the Trans- Mississippi weren't going anywhere. That is one of the falsehoods about armies. While armies are meant to be used against your enemys it is not a wise use of an army just to fight your enemy because he is simply there.
So what was the MILITARY purpose of the Red River Campaign?
Was it to simply defeat the enemy and drive the Confederates from Shrevesport? That is the simplistic view that most take. But a military commander is always looking for the military advantage to be gained. In the case on Shrevesport there was no actual military gain that would be worth the expendature of the Human capital that was to be spent. There was no military advantage to controling the Red River, or southern Arkansas, or northern Louisiana in 1864, or even advancing into Texas. The War was going to be won or lost in Virginia by that time. You weren't worried about tieing down the Confederates so that they couldn't send reenforcement to Alabama, or Virginia. How would they have gotten there?
What was to be gained was MONEY. Remember that it was costing the Union $2,500,000.00 a day.
Shrevesport was in the heart of a very rich cotton growing region and there was money to be made in the sale of Cotton. The Confederates were selling Cotton from this region south to Mexico to be sold overseas and Banks wanted a share of it. He had had his eyes on the region since 1862 and had been formulating this plan since his first arrival in Louisiana. So the Red River campaign was more about money than it was about military necessity.
On the other hand; Most people have the idea that the construction of Fort Hindman at Arkansas Post was about controlling access to the Arkansas and the White Rivers.
While it had that benefit there was ,in my opinion, an even more important Military reason for constructing Fort Hindman where it was. The first part of the Military reason in my thinking was to reestablish a line of communications with the east bank of the Mississippi by providing a safe haven for boats running the Union patrols across the Mississippi and up the Arkansas. And likewise a staging area for boats making the reverse runs.
The second and this was the "thorn" in the federals side, which I believed cause McClernard to act, was acting as a base of operations for the Confederates to conduct harrassment operations against the thinskined supply steamboats that were trying to keep Grants army and Porters navy supplied. The Union did not have enough gunboats to form an effective convoy system for these supplies and land based artillery accompanied by escorting Infantry threaten to close the Mississippi to that traffic. You will remember tha McClernard supposed reason for attack Arkansas Post was the "Blue wing" Incident and a couple of other such destruction of coal barges and such.
Militarily Arkansas Post was built in a far too exposed position for only defending the Arkansas and White Rivers. Why build it were deep water would allow the Union Ironclads to get to it? As seen by the Battle of Arkansas Post, Fort Hindman was far too exposed to assualt by combined Army and Naval forces. Its primary defence may have been it battlements and heavy guns, but it was not intented to stand alone for long without support. Which is where Walkers Texans come into the Picture. However, the Union Navy planned for even this also according to Porter account by sending the USS Rattler to guard the ferry at Jordans Plantation/South Bend to keep Walkers Division from crossing the river and reenforcing Fort Hindman.
Therefore advancing up the Arkansas and the White Rivers after the fall of Arkansas Post was looked at as being "Targets of oppertunity" if the situation presented itself, rather that it being a primary objective of the assualt itself. The primary objective was to eliminate the "Thorn" and secure the Union steamboat supply lines. And while Arkansas Post may have proven to be a good training and moral busting exercise for Vicksburg, that again wasn't the primary objective of the operation.