Petraeus should simply resign.
And the balance of the Generals and admirals.
I think Petraeus would not resign while there are soldiers in the field risking their lives under a strategy he helped develop.
This military does not leave its own men behind.
No because he would be turning his back on his men.
I posted sometime during the campaign that Petraeus wouldn't last six months. Resigned or fired, I hold my opinion. O will not get along with the military nor does he want to. Of course, my all knowing wisdom didn't suspect that he would keep Gates. Wait and see and hope for the best.
Bingo, on point as usual.
Military Commanders can not publicly campaign against the POTUS.
If they feel that strongly about it, they should as you say resign and inform the public that the POTUS is placing politics ahead of policy and the safety of the American people.
If Obambi pushes retreat, he will resign. Look for very low re-enlistment at any rate. And new enlistment numbers should already be dropping now. The military will have to lower standards and/or induce a draft (which the anti-war libs seem to like now that they're above draft age).