He puts his army between Meade and Washington as was his plan.
How? He has one good road due south to get around Meade and though he doesn't know it, at 1PM on July 4th, after a long light rain, the heavens are going to open up on him.
Longstreet tries to disengage and move round the flank of the Army of the Potomac. Whether this move will be successful or not is another thing. The Federals were pretty worn down in the first days fighting, but the Rebs don't have a cavalry screen because a certain Mr Stuart is too busy playing Grand Theft Wagon: Dixie Edition. On the other hand, IIRC at this point in the battle Buford's cavalry was withdrawn and Kilpatrick hadn't been sent to replace him on the left, so the movement may go unmolested.
The Confederates don't have the lay of the land themselves. So they are literally advancing blind. We know where he
should have gone IF he wanted to flank the Union Army. Problem was, where he
actually wanted to go (around the Round Tops) would have been even worse than marching around Meade's right flank. It would have delivered his army piecemeal into the Union Army's line of advance and the ANV would have been of the choice of a pell-mell retreat or having their advance as badly chopped up as OTL's Union III Corps.
There WERE Union cavalry to the south, a provisional cav brigade of VIII Corps (survivors of the fall of Winchester) that was in communication with French. The southern route was not an open door. And Union roads to the south (and the Pipe Creek Line) were better than the ANV's. South of that, and the ANV faces the prospect of not only having the deeper parts of the Potomac River to their backs, but with torrential rains both the fords will be flooded (though the Williamsport ferry and Falling Waters pontoon bridge will be OK) and the Potomac may be navigable far enough upstream for the Washington Gunboat Squadrons to come into play.

And if Longstreet is caught
on the wrong side of the flood waters east of Falling Waters (an unnamed tiny babbling brook converted into a sizable river/lake)...
If the move is made successfully, then the Confederate troops are probably going to be dead tired from a solid days fight and a night march (and somewhat demoralised by retreating from a victory). There's also the problem that some of Longstreet's chaps from his old corps haven't arrived yet, so will need to take a roundabout route if the field is to be abandoned. The Confederates may also run into Sykes/Sickles (I forget which) corps on the road South, as IIRC this almost ran into the Confederates IRL.
The Confederates CANNOT abandon Gettysburg without being seen, AND being seen as to where they are headed. Secret movement ITTL is not an option. Not before the rains begin in earnest. The AotP not only holds the high ground, they hold VERY high ground.
If the ANV is to redeploy anyway, ultimately that will mean Meade has to just basically do an
about-face to send his army back to where he wanted to fight in the first. Meaning his army will in the end have less marching to do (esp. for V & VI Corps). The Confederates CANNOT "run into the Union Army" on a southward march unless they are crazy enough to use the Emmitsburg Road. Otherwise, they have to pull back. The problem is, with Gettysburg where it is controlling seven major road intersections, losing it means that Meade can EITHER easily pull back to the better than Gettysburg Pipe Creek Line (leaving elements of militia and parts of VIII Corps to "defend" an abandoned Gettysburg) OR, as others have suggested, play the hammer to Washington's anvil. And every day, Meade gets stronger.
The ANV had enough food supplies taken from raiding to last up north for months, but their nearest ordnance resupply train was in late June halfway between Williamsport and
Staunton!
The Rebs could delay a day, that means the Union solidifies its force and that a cavalry screen should be in place on the Union left (IIRC). Even if it is under Kill-cavalry, it's still something in the way, and a turning movement becomes less likely to succeed.
The problem is the ANV cavalry that is left prior to Stuart's return has too many jobs that all desperately need to be performed. Stuart didn't just take himself, he took his best subordinates. Against the (mostly) dross that was left, even if "Kill-Cavalry" does his all to screw things up, his better subordinates are in a position to exploit their natural advantages. And because of all the time spent raiding, there had become a much more serious issue of weakened horseflesh for the Confederates.
BTW, the terrain on the Union Left does NOT make for turning movements. The Union Left was the Round Tops, followed by thick trees with heavy undergrowth, and behind all that, VI Corps.
As for where to stop, part of Meade's Pipe Creek line would be a good bet. The ground is perfect for defence and it sits on the Army of the Potomac's communications to Washington. If the Federals don't detect movement during the night, it may take most of the day before they can move on Longstreet's new position. Knowing how strong the position is and that I and XI corps are rather badly roughed up, Meade may not want to assault and may try and turn it instead.
I'm not sure I understand you here?

Surely you are not suggesting that the ANV form the Pipe Creek Line? How? It's in the main line of approach for the whole AotP Army. Trying to "occupy" it for the Confederates would not only be impossible, but much more so for them to try it
in secret.
Longstreets old corps is probably handed over to McLaws, Hill and Ewell probably wont question Longstreet. Both were used to obeying orders under Stonewall, and Hill spent the entire Battle of Gettysburg trimming his nails and counting flowers. (1)
1) Suffering from his venereal disease actually. Not kidding. Whenever battle loomed, his blood pressure went up, causing his symptoms to return. Remember, Spontaneous Generation, not Microbiology, was the rule of the day. Nobody this side of Joseph Lister listened to a certain quack by the name of Louis Pasteur.
Oh, and at some point in the action Dan Sickles will ask several times for clarification on an already clear order, receive said clarification, and then will come up with his own disastrous order anyway.
Could you please clarify this?
You know, it is worth noting that when Longstreet had an independent field command (1863 East Tennnessee) his army was defeated.
By an army commanded by Ambrose Burnside.
Granted, Longstreet was in close to an impossible position because of his superiors' decisions, but still...his decision making in Tennessee suggests he would be relectant to remain on the offensive in Pennsylvania, especially since he would be in command of the ANV on an interim basis because of seniority, be out of telegraphic communication with any superiors, and (historically) had a strong sense of the possible.
I see Longstreet pulling out and heading south; the simple facts that the AoTP certainly had troops on the march and Meade et al uncertain what was going on with the rebels would suggest the ANV might get a day's lead, but I still expect they'd lose much of the rear guard. Meade after one day of battle is not going to be Meade after three, and Longstreet was not Lee.
Best,
Biggest mistake Meade ever made was to allow himself to be led by consensus, rather than doing as HE thought best, which was to attack the ANV on the morning of July 4th. ALL his corps commanders counseled caution.
The Confederates do have a cavalry but it isn't very good without Stuart. But it probably is enough to screen the withdrawal of the army.
When the mud hits, the horses will be unable to give anymore. BUT, said mud stymies Federal pursuit anyway.
The demoralization will be far more about Lee than a withdrawal to better grounds. I doubt the latter would effect them much at all. Longstreet had a good reputation by this time of the war and they would think he would know what he is doing. Federal troops will be tired as well and Longstreet would pick good ground that the Federals would have to attack. Attacking is more tiring than defending.
Longstreet is withdrawing AFTER having dealt the Federals a moderate defeat (goodbye I & XI Corps) and securing his army enough food to get them clear through the summer, fall, winter, next spring, and well into next summer. They really got a haul, and most of it wasn't even the wagons Stuart finally dragged in on the 2nd and 3rd.
In retreat, however, the terrain is
impossible for the Union to attack, not simply "good ground". Even Ambrose Burnside blind drunk on whisky isn't ordering his army to "attack" across a flooded "lake" (the rain storm again) up to eight feet deep and covered by deeply entrenched heavy artillery!
God knows what Sickles would do. He was a completely incompetent malcontent.
Agreed. Which is why Confedwanks keep maneuvering him to command of the AotP!
Since he historically stated he didn't want to fight at Gettysburg and about it with Lee I agree he isn't going to fight there when he has the ability not to. He is going to get between Meade and Washington on the high ground somewhere and let Meade attack him. I don't know how much of the rear guard he would lose as it would only need to be a delaying action.
Problem: THERE ISN'T ANY "WHERE" FOR LONGSTREET TO GO! There IS NO high ground location that he can take, or even reach, in time before the Union Army gets there first. Interior lines is what you get when you are the defender. Which is what the AotP was in this campaign. Lee himself kept carrying on as if he were still in Virginia, with all the advantages that brought him.
If Longstreet digs in at Gettysburg, waiting for Meade to attack, he's got a long wait. With Vicksburg happening just a couple of days away, Lincoln's not going to fire a new hire so quickly. And Meade has time on his side.
I mean, consider - the one truly sucessful commander the ANV has ever had is dead on the field; the man who suceeds to command understands how strong the AotP is in the abstract and specifically on the chosen field; he knows Meade is not the man that (say) McClellan or Burnside was; he knows Stuart's cavalry is out of contact, but Meade's is; and he knows there's a window of about 24 hours where the ANV can pull out to the south, or face the reality that they may not pull out ever (forget going west or east; crossing in front of an established position is not a good move).
Agreed with all. And going west only gets you to the rolling hills of Hancock, and eventually the impassable Allegheny Mountains.
Oh, and Longstreet - generally seen as a solid subordinate who is the soul of prudence - is in his first independent command and out of communication with his superiors. I really doubt he is going to head deeper into enemy country.
His history as an independent commander does not inspire confidence.
He has the army's artillery, what passes for their supplies, and the wounded from the first day to protect/withdraw - I really don't see Longstreet staying in Pennsylvania a second longer than it takes to organize a retreat.
Best,
Not just "passes" for their supplies. See Above about all the (edible) loot they were taking home. They were better fed than at any other time in the war IMO.
And think of all those PoWs from (by this point ITTL) I & XI Corps.
If his post match commentary can be taken seriously then he will try to disengage and move south to get between Meade and Washington. Bare in mind Meade was also new to command and his pursuit of Lee was lacklustre.
As mentioned by
TFSmith121 below, Meade was appointed to command, Longstreet in mid-battle by the death of his commander. Meade is in a better position in terms of confidence. You can blame historic rains on his pursuit AFTER July 4th. That day is on him.
You know that Pipe Creek line is interesting. I wonder if any of the Rebels knew of it and was aware of its defensive value?
There's no evidence of that. The Confederates had no available cavalry in an area swarming with Union forces, and the Union decisions regarding the Pipe Creek Line were relatively recent for any espionage efforts to be of use.
Meade's pursuit of Lee was hardly 'lacklustre'. The Army of the Potomac was very badly worn down by the fighting, the conditions were awful, and by the time the two armies could come to grips again the Rebels had dug a strong defensive position. To have attacked would have resulted in a bunch of Union dead for little gain.
Agreed absolutely.
Moving between DC and the AotP is actually a really bad idea when I think about it.
Longstreet would be placing himself between the insurmountable ring of fortification surrounding the capital, and a larger enemy, trapped in enemy territory and very prone to be surrounded and put into siege himself.
He basically has to count on a political reaction on the union side which forces the AotP to mount a pre-mature attack on his position. Which admittedly might happen.
Over Meade's dead body. He destroyed himself in late 1863 in Lincoln's eyes by refusing to launch suicidal attacks in the Bristoe and Mine Run campaigns.
For the South to win they have to go all in...even then it's Lincoln and he's not likely to give in if he still has options.
Isn't "go all in" pretty much a good description of what they did? The South mobilized 90% of their available military manpower, and that's factoring in White Southern Unionists. Take them out, and its as close to total mobilization as makes no difference.
The North only mobilized 50%, including militia, short-term enlistments, draftees, and substitutes.
At least Meade had been appointed as army commander by his commander-in-chief.
Longstreet would be taking over - in the middle of a campaign - in the midst of battle - on an actual battlefield.
And his commander-in-chief wouldn't even know Lee was dead, much less that Longstreet had taken command...
A West Pointer is not going to try and switch gears in the middle of someone else's campaign, in the middle of a battle he was not keen on fighting anyway, and without some communication with his superiors.
Just not going to happen. There's a reason the US Army had professionalized itself since the Revolution and 1812-15 war.
Beyond that, disengaging and withdrawing south - complete with army-level supplies, artillery, and the wounded from Day 1 - is going to be difficult enough.
Especially without any cavalry.
Which Meade has, both at hand and on the move...
Disengaging and moving laterally to the east - across the face of Meade's rapidly growing forces in an excellent defensive position, and with various additional US forces marching west - would be damn near suicidal.
Meade wasn't Grant, but he sure as hell wasn't McClellan or Burnside, either.
And Burnside, to be fair, beat Longstreet pretty soundly in east Tennessee later that same year...
Best,
Agreed with all. Burnside was a disaster as an army commander, OK as a corps commander, quite good as a divisional commander. Were he and John Bell Hood switched at birth?
