Given what General Hunt wrote, he was already biased toward defending at Gettysburg so it makes sense that he would agree with Sickles. But he wasn't Meade, and Meade wanted defended what he viewed as the overall best defense. It wasn't up to Hunt or Sickles to second guess and countermand Meade's orders. When you're given an order your only response is, "yes sir, yes sir, three bags full", and carry out your orders.
Of course! But from Sickles' point of view, he's concerned about his position, Hunt has concurred, and Sickles has sent someone (Hunt) to ask Meade about it, and no response has come back. So it seems like his concerns have been ignored.
If Meade hasn't actually inspected that part of the line (which is my understanding) then as far as Sickles knows the position has been picked by Meade off the map without actually confirming that the ground is good. This is almost exactly the situation Sickles was in at Chancellorsville, and at Chancellorsville he got the **** pounded out of his force (and this was a key part of the Federal defeat).
What I'm trying to do is to explain what was going on in Sickles' head. This is one part of exploring his decision; looking at what
Sickles thinks would happen if he did not advance, and whether Meade is aware of the danger.
Sickles has reason to think there is danger, and he has reason to think Meade is unaware of it. Under these circumstances, Sickles could reason that moving out to the advanced line would prevent a defeat.
The second part of the question - the legality of what Sickles did - is pretty clear. Sickles was ordered to take up a position, and the extent to which he should not have advanced is exactly related to the extent to which you feel a commander has latitude to act beyond their orders (which is known as discretion). If you feel a commander has no discretion, then obviously he was completely unjustified, but the Civil War is replete with commanders acting outside their orders (including actively disobeying them, something Jackson did on more than one occasion) and garnering success.
It is generally the case historically (Napoleonic Wars) that corps commanders are expected to exercise at least some discretion, but the Napoleonic corps is not the same as the ACW corps.
The third part of the question is whether what Sickles did actually did improve things for the Union, and that is much harder to tell because you can't really rerun the battle over. It looks however like if Longstreet's attack had gone home on Sickles' original position he would have been in trouble anyway, and there is the possibility of the loss of the road.
This means that my view is that Sickles was:
- Justified according to the information he had and the view he had - probably. He had good reason at least to believe that he was doing something that would avert defeat and that doing so without orders was a reasonable course of action, even if not legal.
- Justified according to the law - this depends directly on the limits of corps commander discretion. Sometimes they're expected to have a lot more than they are other times - the classic case of the commander who hears the sound of guns and rides to them is applauded, but in that situation a commander who sat in place would be the one obeying orders. McDowell during the Northern Virginia Campaign exercised his discretion to block Thoroughfare Gap, without orders, and nearly saved the whole Union operational position.
- Militarily justified according to whether his actions materially improved the Union chances of success - this is a tricky one. I think you can argue that Longstreet hitting 3rd Corps in their original position and without reinforcements en route would have done more damage to the Union position, but it is difficult to disambiguate this from the chance hit that brought Hood down in our timeline and misaligned his whole division.
However, Hunt went on the explain the problems associated with this “new line”, and reported back to Meade. I guess it depends on what you mean by Sickles’s advance, but Graham moved out to the PO in response to the Pitzers recon.
I mean the general advance by Sickles' whole force. Unless the timeline is a lot more compressed than is my understanding there was ample time for Hunt to have reported and for Meade to have come and checked the situation out himself - I've seen mention that Meade didn't want to move any of his corps until 6th Corps arrived, but unless he was actively doing something else important Meade could presumably have come and checked in on Sickles.