The trans-mississippi theater of the war is almost entirely forgotten and neglected by modern histories. The reasons for this cannot be pinpointed to any one case, but there seem to be three that offer a good explanation.
First off, practically everything in the war gets overshadowed by the Virginia theater. It's where all the big-name battles like Chancellorsville, Fredericksburg, and Gettysburg were fought. It is also where the most famous commanders, Lee and eventually Grant, squared off. It is also the site of some of the worst bloodletting.
Second, the very nature of the trans-mississippi battles was such that they tended to occur at coastlines and borders where a yankee invasion force was repulsed. Because of this the yankees never really got a foothold in this theater, meaning there were no great inland battles between massive armies beyond those that halted the invasions in their infancy. Naval battle sites are, for obvious reasons, difficult to mark with monuments and historical parks, meaning you can't "visit" the battle of Galveston in the same sense you can "visit" Lookout Mountain. This makes their presence less known and less immediate to the people of today.
The third reason that the trans-mississippi theater is neglected seems to be political. Of the war's three theaters, it was the one where victory after victory after victory went to the confederates. Despite numerous well-funded and heavily supplied tries the yankees kept getting kicked out at the coast or border in some of their most lopsided and unlikely defeats of the entire war. Since modern historians are keen on portraying the south as evil, and since they are keen on presenting some of the north's nastiest, cruelest, and even in some cases the most incompentant of the northern commanders as "heroes" of the union. That doesn't fly well with the trans-mississippi theater, where every time any commander of any number of yankee troops and ships tried to invade the Texas coast, they found themselves repulsed by the the fiery irishmen of the Davis Guards and the converted civilian steamboats that rammed and captured fully outfitted ships of war.
From time to time you will likely encounter some, such as FR's Wlat brigade, who attempt to downplay and belittle this theater of the war, most often due to their own subscription to that last reason I stated. Such claims are utter nonsense considering the size of the expeditions that were repulsed in that theater.
The Red River campaign had some 45,000 troops committed to it plus the largest inland fleet of warships ever assembled, 58 in total, on the north american continent up to that time. They loaded up ironclads and warships to navigate up river and converge in northern Louisiana for the invasion of Texas and the confederacy's cotton reserves contained within it. Lincoln had personal stakes in the attack as he needed the cotton, needed to rob the confederates of it, and desired strongly to establish a union presence in Texas. An inferior number of confederates, roughly half the union strength, charged the yankee line at Mansfield and overwhelmed it. It turned the tide on the Red River campaign, which withdrew in defeat shortly after the battle.
Though a one sided against the odds confederate victory, Mansfield does not even begin to compare with what happened at Sabine Pass. In another of Lincoln's attempts to invade Texas for the same purposes, this time by way of the gulf, the yankees assembled a fleet of two dozen warships to converge on the Texas border, sail through the pass, and invade from the east of the port of Beaumont. Standing in their way at the pass was an earthen fort with six cannon. It was manned by 44 irish dockworkers under the command of a single lieutenant. They were virtually all that stood between the yankees and the landing point. The yankee fleet, carrying 5,000 soldiers on hand with some 20,000 more to follow after them, entered the pass expecting virtually no resistence. As soon as the ships came into gun range though, the tiny confederate garrison commenced firing under directive to make every shot count. The first two ships in range were hit dead on with precision and immobilized with massive casualties. The next two were run aground and the remainder set in for retreat to New Orleans. The confederates suffered not one casualty in the entire battle.