The first record I have found of Scott's Brigade (formed at Demopolis, Alabama) is dated 10 MAR 1864. It was composed of four regiments: the 55th Alabama Infantry, 57th Alabama Infantry, 9th Arkansas Infantry and the 12th Louisiana Infantry. Colonel Thomas M. Scott, formerly Colonel of the 12th Louisiana Infantry was the new brigade commander. Loring's Division including Scott's Brigade departed Demopolis for Montevallo on 2 APR 1865. When Lieutenant General Leonidas Polk's Army of Mississippi was ordered to join the Army of Tennessee, Scott's Brigade lead the way departing Montevallo by rail on 5 MAY 1864 and reaching Blue Mountain the next day. They then marched overland to Rome, Georgia. General Polk, at Rome with the lead elements, was ordered by General Joseph E. Johnston to procede immediately eastward and take command of all forces at Resaca. Their arrival by rail at Resaca on 10 MAY 1864 is usually taken as the date on which Polk's Army of the Mississippi joined the Army of Tennessee.
Thomas M. Scott's promotion to brigadier general, and Lieutenant Colonel Noel Nelson's promotion to Colonel of the 12th Louisiana Infantry were announced in June 1864, but were to be effective for rank and pay as of 10 MAY 1864.