What would have happened if instead of launching the Gettysburg campaign, the Confederates took a defensive stance in the Eastern theater in order to send forces to help relieve the pressure on Vicksburg?
They must have seen the the pressure Grant was building on Vicksburg a mile away, couldn't they have detached a portion of Lee's army to help hold it while taking a defensive stance in the Eastern theater?
This is the biggest issue: they couldn't. A proposal to send reinforcements to Vicksburg before Grant's Third Campaign would have looked at skepticism. For all intents and purposes, Grant's efforts to seize Vicksburg up till May have been utter and complete failures. The final route which Grant took seemed impossible. While there was some reports and a skirmish showing the existence of Grant's army on the Louisiana side of the Mississippi, they could not confirm its strength and intelligence reports suggested that Grant was in fact withdrawing to Tennessee to help Rosecrans. Hell, Union high command didn't even know what Grant was doing and most of Grant's subordinates were begging him not to carry out the Third Campaign.
It's not until Grant suddenly blitzes the relief force and besieges the Army of Mississippi in Vicksburg that the need to relieve Vicksburg becomes apparent. Even then there was the Army of Relief under Joseph Johnston, composed of 30,000 men from the Army of Tennessee, Loring's Division, and several brigades from the coastal garrison. So Lee could have argued that his force was not needed. That said...
Historically, Seddon requested that a division or two of Longstreet's Corps be sent to join Johnston's Army of Relief. They could in theory do so, but in my opinion it would be a fool's errand. Due to the lack of direct rail lines, for a Confederate force to move to Vicksburg from Virginia, the detachment would have to go from Richmond to Atlanta, then through Alabama and then to Jackson. Given that it took Longstreet's first units 4 days to reach Atlanta (and nine for 15,000 men) and it took Walker's brigade 6 days from Georgia to Jackson (May 5 - May 11), a division might arrive to Jackson, Mississippi after 12-14 days of travel* at minimum. The number of days would almost certainly double if you send Longstreet's entire corps into Mississippi.
So let's say that Lee has to accept the transfer of some troops to Vicksburg on May 17, the end of the conference between Lee, Seddon and Davis about the Vicksburg situation. This was the day they agreed to launch the Gettysburg Campaign to draw Grant from Vicksburg. Lee sends:
Let's say it's Pickett's. Pickett's 5,000 Virginians could probably arrive at the start of June, just as Breckinridge, the last of Johnston's OTL reinforcments, arrives. This brings Johnston's strength up to something around 36,000 men (although Johnston would probably insist that it's only twenty-nine thousand). However, would Johnston actually attack now? Despite having 31,000 men by June 1 IOTL, Johnston constantly demanded reinforcements. Is 5,000 men enough to convince him to make an effort?
Another thing: Grant was increasingly prepared to repel Johnston. On June 8, Grant reported that he could muster a force of 30,000 men to repel Johnston while maintaining the siege. The approaches to Vicksburg by land are very finite and Grant ensured that all of these approaches had been fortified and manned. Grant's right flank was firmly anchored on the Big Black River; in addition, Grant positioned his forces so the rich farmlands in the area that could have fed any relieving Confederates armies seeking to operate there were not accessible to the Rebels. Grant had also prepared extensive defenses for a potential Confederate approach from the Mechanicsburg corridor from the other flank, which was the most practicable approach for Johnston's forces.
I don't think that Johnston would do anything even with this reinforcement. This would just be a distraction of Confederate manpower as Johnston's Army of Relief really was.
Sending a majority of Longstreet's 20,000+ men to Mississippi would give Johnston a very formidable strength of 50,000 men. But the time taken would have been immense: possibly a month. By June 17, Grant too has received reinforcements: he has Francis J. Herron's Missouri Division on June 11 and John G. Parke's 8,000-man strong IX Corps on June 15. In a letter to McClernand, Grant notes that in the case of an extreme emergency, Grant could strip out the siege line for more manpower. Once again, I doubt that the Army of Relief could succeed in breaking Pemberton out.
On the Eastern Theater, things will be interesting to say the least. There are major costs and benefits to staying on the defensive.
Staying on the defensive will give the Union the strategic initiative for a second summer campaign. However, with the expiration of 2-year enlistments and the whole toxic command atmosphere with Joe Hooker, the Union might be forced to reorganize, which delays their offensive.
Assuming that only a division is sent, the situation wouldn't be too bad. IOTL Pickett actually left behind 2 brigades to defend Richmond during the Gettysburg Campaign. So even if Pickett goes, Lee still has two brigades to make up for the loss of Pickett's Division.
A major cost of having no Gettysburg Campaign is the loss of the forage gained in rich Pennsylvania. War-torn Virginia needed time to recover and refill their stores with supplies of food. So in the aftermath of the Gettysburg Campaign, Lee had taken more than 45 miles of quartermaster and subsistence trains filled with impressed stores escaped Pennsylvania and Maryland along with 20,000 horses and mules seized and large herds of livestock. Throughout the rest of the summer Lee's men had enough meat, flour and fodder. All of which were unavailable in Virginia. This will make his supply situation ITTL much worse then in OTL.
So, overall, I think that the big issue with a detachment to Vicksburg is that the Confederate logistics is lacking in capacity to transfer Confederate troops from Virginia to Vicksburg quickly enough. The window of opportunity to break Pemberton out was probably late May, while the XVI Corps was transferring troops to Vicksburg. Staying on the defensive in Virginia might be the tactically safe choice, but it would severely worsen the Confederate supply situation in Virginia. There really isn't a good option for the Confederates at May-June 1863. There's Longstreet's proposal to send his corps to join Bragg's Army for a drive on Nashville, but I think the logistical buildup would have taken too much time to accomplish to help Vicksburg or Virginia.
*: It is important that I mention that I am assuming that the winding railroad path from Georgia to Mississippi has the same carrying capacity as the line from Richmond to Atlanta and I've seen suggestions that it's not. Of the two brigades sent from the coastal garrison, it seems that even after nine days of travel, only Walker's brigade and half of S.R. Gist's brigade made it to Jackson in time for the battle. So it took nine days to transport IIRC almost three thousand men. Not a very positive sign to say the least.
It's a long way from Virginia to Mississippi (literally 1000 miles from northern va to vicksburg going through A LOT of Appalachia). What's the largest functional armed force that has ever marched that far that fast?
It's a long way from Virginia to Mississippi (literally 1000 miles from northern va to vicksburg going through A LOT of Appalachia). What's the largest functional armed force that has ever marched that far that fast?
Of course, they won't marching there. The CSA does have railroads to carry the troops from Virginia to Jackson, Mississippi. The big problem is capacity. Now that I've had time to re-read my copy of "The Chickamauga Campaign: A Mad Irregular Battle" by Dave Powell, I realize that I forgot/should highlight several facts:
Confederate rails from Richmond to Atlanta could only handle 3,000 men at most, not including the supply wagons. In the nine days it took to transfer Longstreet's Corps, not a single wagon was transferred. This resulted in Longstreet's troops badly starving (not to death) at Chickamauga-Chattanooga. If we include the wagons for the Mississippi journey, then the bulk of Longstreet's Corps could only arrive at the end of June at best (and I'm being very generous).
As Longstreet's Corps arrived to Atlanta, bottlenecks at Atlanta arose. Since reinforcements and supplies flowed on the same railroad, regiments had to wait for days for their turn on the railroad. With this fact, I'm beginning to think that it will require more than 14 days to transfer a division of 5000 men from Atlanta to Jackson and that a corps will simply get there too late.
That said, I think that the best realistic option for the Confederates at Vicksburg is to evacuate after the disaster at Big Black River. Even Grant in his memoirs acknowledges it so. Simply put, there was no way for the Confederates to gather enough troops in time or withstand more than 47 days of siege. Even if Pemberton had the food, Grant was fully prepared to make the Petersburg Mine look like a harmless prank by detonating 13 mines along Pemberton's defensive line in a prelude to the grand assault on July 6.
Had Pemberton opted to give up Vicksburg on May 18, he would have to give up the thousands of wounded and sick as well as the coastal batteries that protected Vicksburg from the navy and might suffer from straggling (his army and staff was inexperienced at organizing and conducting marches). However, Grant would have had to go for Vicksburg first. He needed a day of rest to finally get his army some proper food and refill the wagons. There's admittedly a risk that Grant could intercept Pemberton, but I don't think that it would have resulted in their complete destruction. Most importantly, if there's no siege of Vicksburg, there's no reason for the Army of Relief. This may have butterfly effects in the Tullahoma Campaign, where the Confederates have greater strength. Then again, there's also the possibility that Bragg squanders his greater strength.
If any forces are sent west from Virginia in May-June, they will not be following Longstreet's OTL September route, but will rather follow the more direct rail line from southwest Virginia through Knoxville and on to Chattanooga.
I don't know how the capacity of this line compares to the more roundabout route through Atlanta, but if nothing else it is a significantly shorter distance. Just something to consider if we flesh out this scenario.
I agree with everything above regarding the impossibility of transferring units from the ANV to the Vicksburg garrison. However, there were plenty of opportunities for the Vicksburg campaign to go differently & I might have a crack at a timeline one of these days.
