The factor you mention is neither new, nor something left unaddressed by military people who have been contemplating what would be necessary for a successful counterinsurgency since the Eisenhower presidency.
Fuel is a insurmountable problem in some parts of the world, but not here.
How many mobile semi truck refining facilities do we currently have? I mean there must be a whole lot of them since oil just flows right out of the ground and everyone can do it..........that's why gas and diesel are selling for around .25 a gallon right now?
And of course we can transport fuel from our current wells to refining semis and forces in the field without using fuel consuming trucks, or traveling long distances? And of course all of the manufacturing infrastructure that will allow us to make spare truck parts, drilling equipment, and refining equipment will all remain in place after the SHTF? Oh, and let's not forget a sizable portion of your force can also be supplied and managed to secure all of that infrastructure.
Fuel is just one aspect of a military's logistics train. You also have BBB.....beans, bullets, and bandages considering that ALL of those things need to be delivered in a relatively quick manner, that means they most likely need to be delivered on a truck. Not to mention the need for trucks if you're going to be moving your howitzers around, or your troops with any amount of speed.
And yes, this idea is nothing new; however, let's take a look at Iraq......we've been engaged in COIN since 2003 - that will be 10 years this March.
Let's take a look at Afghanistan.....we've been engaged in COIN since 2001 - that is just over 11 years.
And operations within both of these countries are being fought by multinational coalitions with the latest and greatest weapons and gadgets, with some of the most advanced logistics trains in the history of mankind. Oh yeah, did I mention these are PAID VOLUNTEER FORCES, which have historically shown to be more effective than conscripts. Iraq and Afghanistan are each, what the size of New York? What about the other 48 states? (or 55 if you're the incompetent CiC we have now). What about contingencies such as mass numbers of refugees flowing north from Latin America?
Just as a small example......try reading up about the difficulties of getting fuel and logistics around in Afghanistan; Google "NATO fuel trucks attacked Afghanistan." Now, multiply that by 50 more states, while accounting for a large portion of your force deserting, as well as a break down in infrastructure such as the production of food, ammunition, and medical supplies.
I highly doubt that the continental US could be reconquered under such circumstances following a full out no kidding SHTF scenario without major use of nuclear weapons....and even then, the county would be vastly different than it is now and most likely STILL could not bring pockets of resistance in deep rural areas under control.
Hell even in the War Between the States, the Confederates almost won and they barely had means to manufacture cannons at the start of the war. Think of all the machine shops in the country now that could make cannons or black powder mortar systems relatively easily......hell you can buy them on the internet now for about $400.
It is much more plausible to see thee country break down into many smaller fractions. And in order for that to happen, it would be MANY fighting forces and NOT TWO LARGE OPPOSING FORCES SUCH AS WWII OR GULF WAR I, which was what you originally stated would happen; something akin to WWII or GWI vice Afghanistan or Bosnia. And even after that, there would still be pockets of violence/resistance for a while after that (Shay's Rebellion following the Revolutionary and the War Between the States [as predicted in the Anti-Federalist Papers]).
Things are a lot different now and commonalities don't necessarily span such large areas like in CWI; warfare has also evolved since then too, it's also why we're not manning trenches in Iraq and Afghanistan like in WWI and WWII.
Any prepper should prepare to NOT be relying on oil and gas. Wood, solar, maybe wind, and human generated electricity are the only ways, hydro-generation would be great, but rare.