Seems that the seven Trans Mississippi regiments wanted for service east of the river evolved from going to Richmond, to going to Vicksburg. And, as you know, never making it to either place.
I believe the initial motivation for sending these troops east stemmed from Lee's growing admiration of the 3 Texas regiments in his army [and of course the 3rd Arkansas regiment]. Lee wanted to organize two brigades of these westerners.
R. E. Lee: A Biography by Douglas Southall Freeman
A vigorous effort was made, at the same time, [Fall 1862] to strengthen the Texas units, which had now become Lee's favorite shock-troops.
Lee wrote General Lewis T. Wigfall: "I rely upon those we have in all tight places, and fear I have to call upon them too often . . . with a few more such regiments as Hood now has, as an example of daring and bravery, I could feel much more confident of the campaign"
In a letter written to General Wigfall only one day after the Army of Northern Virginia returned across the Potomac on September 20, 1862, General Lee paid the Texans perhaps the greatest compliment ever: "General, I have not heard from you with regard to the new Texas regiments, which you promised to raise for the army. I need them very much. I rely upon those we have in all our tight places, and fear I have to call upon them too often. They have fought grandly and nobly, and we must have more of them. Please make every possible exertion to get them on for me. You must help us in this matter. With a few more regiments such as Hood now has, as an example of daring and bravery, I could feel more confident of the campaign."
OR Vol 13
Richmond, Va., October 15, 1862. General T. H. HOLMES, Little Rock, Ark.: Send seven Texas regiments from your department to the army in Virginia. They may be unarmed, and need not be sent until your immediate operations are concluded
10/16 HOLLY SPRINGS, Lt Gen PEMBERTON: ....The enemy in West Tenn is about 45,000 strong, and reinforcements daily arriving. You had better get some of Holmes' troops, if you would save Miss. VAN DORN
10/18 Holmes to Hindman
…I returned this evening from Austin [Arkansas]. The division there is very much crippled by disease, though I think it will be ready to march as soon as we can get guns, or immediately, if you are hard pressed…..
If the division at Austin comes up I will accompany it, for it will be putting all on the cast and I will be present at the throwing. To complete my depression I have just received a telegraphic order to send seven regiments of Texas troops to Richmond. Let this be entirely confidential. I will probably send Garland's three regiments and Sibley's brigade.
OR Vol 13
HEADQUARTERS TRANS-MISSISSIPPI DEPARTMENT, Little Rock, Ark., October 19, 1862. (Received November 22.)
General S. COOPER,
Your telegraphic order to send seven Texas regiments to the Army in Virginia is received. I will order Garland's brigade of three regiments, now at the Post of Arkansas [Arkansas Post], and Sibley's brigade of four regiments, believed to be now reassembled at Marshall, Tex. I have ordered Major Bryan, assistant adjutant-general, to proceed at once to Marshall to supervise and hasten the movement of the four regiments from that place, General Sibley having been ordered to report in person to General Hebert at San Antonio, where the charges against him are to be inquired into. Colonel Garland will move with his brigade as soon as the guns are mounted in the fort now being erected at the Post of Arkansas; say in ten days. The enemy have quietly collected a large force in Northwestern Missouri. I have sent General Hindman to take command and aggregate all our forces in that direction. It may be that he will require the division that is now in front of Helena; if so, I will accompany it and direct the operations in person.
TH. H. HOLMES,
Volume 15
HEADQUARTERS TRANS-MISSISSIPPI DEPARTMENT, Little Rock, Ark., October 18, 1862. Brig. Gen. P. 0. HEBERT, Commanding District of Texas, San Antonio, Tex.: GENERAL: I am instructed by Major-General Holmes, commanding the Trans-Mississippi Department, to say he has ordered Maj. G. M. Bryan, assistant adjutant-general, Confederate States Army, to Marshall, Tex., for the purpose of organizing the four regiments of Brigadier-General Sibley’s brigade, in order that they may be sent to Richmond with as little delay as possible.
The general desires that you will give all necessary orders to facilitate Major Bryan in the discharge of the duties thus assigned him and the speedy forwarding of these troops, which will be sent to Richmond, notwithstanding any order you may have received to send them to Louisiana.
If four regiments cannot be organized immediately in General Sibley’s brigade you will designate four others to go unarmed, as that number must be sent from your district, and at once. A copy of the instructions to Major Bryan is herewith inclosed. I am, general, very respectfully, your most obedient servant, S. S. ANDERSON, Assistant Adjutant-General.
HEADQUARTERS TRANs-Miss DEPARTMENT, Little Rock, Ark., October 18, 1862. Maj. G. M. BRYAN, Assistant Adjutant- General, Little Rock, Ark.: MAJOR: I am instructed by Major-General Holmes to say you will proceed forthwith to Marshall, Tex., and superintend the organization of the four regiments of Brigadier-General Sibley’s brigade.
To this end you will draw on the conscripts for the purpose of filling up the ranks of those regiments, and as soon as they are organized they will be sent, under the command of their respective colonels, to Richmond, Va., to report to the Adjutant and Inspector General of the C. S. Army. Each regiment as it is organized will be sent off without waiting for the others. These troops will be sent unarmed. Having performed the duties above assigned, you will return without delay to these headquarters. I am, major, very respectfully, your obedient servant, S. S. ANDERSON, Assistant Adjutant- General. P. S.—If the regiments are mounted they will leave their horses and arms, to be remounted and armed on their arrival in Virginia. Full discretion is given you. Send four regiments as soon as possible. TH. H. HOLMES, Major- General.
About now, the destination for these troops was shifting from Richmond to Vicksburg.
HEADQUARTERS TRANS-MISSISSIPPI DEPARTMENT, Little Rock, Ark., October 24, 1862. Brig. Gen. P. 0. HEBERT, Commanding, San Antonio, Tex.: GENERAL: The order for the movement of the Texas regiments (Sibley’s brigade) to Richmond, directed in my letter of the 18th instant, I am directed by Major-General Holmes to say is countermanded. Major Bryan has been advised to that effect.
10/30 Pemberton to Van Dorn- I have been notified by the Secretary of War at Richmond, by telegram under date October 28, that seven regiments have been ordered from General Holmes' army for General Lee in Virginia, but that I am authorized to detain them. The regiments are stated to be unarmed, but I am notified that arms are being sent from Richmond for them. I propose to place them under your command. With this increase of infantry strength I think it will be well to allow the Texas regiments to be remounted. The cavalry brigade organization you can continue. If you have more cavalry than you need I have other points where their services are greatly wanted. Let me hear from you on the subject.
11/13 Pemberton to COOPER, Sec of War informed me by telegraph under date 28th October that I was authorized to detain seven regiments unarmed Texans which had been ordered to General Lee, and also that arms were being sent me. Up to this date I have heard nothing further of either the troops or arms. I am greatly in need of both. In fact, I have now nearly 5,000 unarmed men.
As to another poster's reference to Garland, he was indeed captured at Arkansas Post. But, after release from prison, he seemed to be a convenient target of the blame for the surrender, and was without a command. He was not in command of any troops surrendered at Vicksburg.