1st US Army / XII Corps

[Adm.Lee]

Short answer: Field Army headquarters exist to run more than two corps. The rule of thumb is that a commander can keep track of 3 to 5 subordinates. If he's got more than 5 units under him, it's time to insert another layer of command & control.

FWIW, the "prewar" US Army had eight Armies (a.k.a. "Field Armies"). First through Sixth were regional organizations, meaning they were the administrative headquarters for all (or nearly all) Army assets in their region. I'm pretty sure that included training the Guard and Reserve units.

As I understand it, in the field, an Army is considered more permanent than a Corps (or, at least it was), and is supposed to handle more of the administrative, logistical, and support functions, such as pushing supplies forward to the divisions and attached formations, while the Corps HQs are supposed to focus more on tactical issues. I don't know if this doctrine is carried out anymore. A lot of deep rear-area functions, like post office units or laundries, are assigned to Armies, but not to Corps.

Having said all that, IMO, the JCS in Colorado would try to keep at least something in existence named First Army, to run Army operations in the (North)east. To me, that sounds like whatever's left of the 78th at Fort Dix, plus that post's cadre and whatever else is around. If it's at Richmond, it's well-placed to receive and deploy the returning guys from Omega. (Note I said well-placed, which isn't necessarily the same thing as well-prepared.)

I now have a hankering to look and see where the other Army & Corps HQs are in Howling Wilderness.