The transfer of the Army of the West from Arkansas to Mississippi is discussed in what I like to think is a clear and lucid manner in Chapter 15 of William Shea and Earl Hess, PEA RIDGE: CIVIL WAR CAMPAIGN IN THE WEST.

In a nutshell here is what happened. In February 1862, Beauregard urged Van Dorn to join the great Confederate concentration at Corinth that was going to recover Tennessee and liberate Kentucky. Maybe conquer Ohio and Nova Scotia too. Van Dorn declined because he was about to embark on the Pea Ridge operation, which he believed would carry him to St. Louis in triumph.

After his defeat at Pea Ridge, Van Dorn belatedly agreed to join Beauregard, but it took weeks to regroup after the battle and the awful retreat, more weeks to slosh across central Arkansas during the spring rainy season, and a week to paddle down the White and up the Mississippi to Memphis, then on to Corinth by rickety railroad (was there any other kind in the South?). So Van Dorn arrived after Shiloh. Nobody blamed him for that, as he had not been issued any sort of timetable or deadline.

He was blamed, and rightly so, for abandoning the entire Trans-Mississippi District (Arkansas, Missouri, Indian Territory, north Louisiana)to the Yankees without getting approval from the War department or anyone else in authority. The Confederate war effort in the T-M never recovered. Tom Hindman had to start from scratch to rebuild.