Let us take a moment before we begin giving the AI its objectives to explore the interface for divisions, how you can move up and down the chain of command at will.
Click on any unit on the map. The result should look somewhat like this:
Notice the bars on above the unit, these are the various 'layers' of command above our unit. I have intentionally selected a lower tier unit to demonstrate. What we have here is 11. Infantrie Division, a single division consisting of three brigades of infantry. The division is the smallest strategic level land unit you can control and will consist of 1-4 brigades. You can see that 11. Infantrie Division is part of 1. Armeekorps, which is, as its name states, a Corps. This Corps would, in HOI 2 terms, be a small army consisting of three infantry divisions much like the one we have selected. Above 1. Armeekorps we have 3. Armee, an army, which has two other corps like 1.Armeekorps, as well as several individual divisions in its reserve.
We can tell all of this by looking at the list on the right, in the summarizer, in case your not following where I get all this from.
3. Armee is one of the armies that is part of Heeresgruppe Nord, an army group, which has several other armies like 3. Armee. Heeresgruppe Nord is itself one of two army groups that are under the theater command of Ober. der Wehrmacht, which is in charge of the area in red. Let us see what happens if we click on Heeresgruppe Nord.
We have moved to Heeregrupe Nord, which is itself a division consisting of 1 brigade, the HQ brigade, under that brigade we see a short list of the corps it controls as well as the divisions it has in its reserve. Notice how the unit itself and the corps under it are seperated in the display. These corps are not in the same 1 brigade division as our army group HQ, after all.
Now, let us learn how to detach and attach units from their command structure. Let us click on another unit.
Here we have 252. Infantrie Division, one of the divisions that Heeresgrupe Sud has as its reserve divisions. Notice the button I have circled in red and drawn an arrow pointing to. If we click on it, this division will fall out of the command structure and will be a freefloating piece of flotsam.
Let us make them feel less lonely by reattaching them to the command structure.
Notice how 252. Infantrie Division is listed as its own entity on the summarizer on the right, being neither under OB West or Oberk. der Wehrmacht. If you turn on the AI for both of these theaters and give them objectives, this division will not be affected by them, will not take order from them, and will sit in place until *you* give them orders. In the offensive that is about to develop, we could detach a division, an army, or an entire corps from the chain of command and command them directly, or give them a specific objective under AI control to, say, close a developing pocket if we wished, and then reattach them to the command structure when we're done.
Note that you cannot give orders to a unit that's under an AI controlled HQ. However, if you detach your unit, give it some orders, and then promptly reattach it, the AI will not interrupt those orders, as far as I can tell. I've experimented with this for a bit now, and it seems to hold true. YMMV, of course. Still, if what I have seen on my own is at all normal, it should be quite possible to close off any developing pockets without the AI mucking it up, if that is your desire. Note that I said the AI won't interrupt *orders* not objectives, which it *will* override. You'll want to order the attacks yourself and then reattach the units in question, not merely set objectives for the AI. If you want to set objectives, you'll want to leave the HQ AI unattached from its parent until the objective has been completed.
If we remember where they came from we can reassign them there, or if we have forgotten we can simply attach them as reserves to the nearest AI controlled HQ, who will manage them within its own objectives. The tooltip will only list HQ's of the appropriate level (it won't let you attach an army group to a corps) and will have the nearest HQ's near the top. Remember that AI objectives always flow down the hierarchy from the top, and that the AI will never change the hierachy you've set up.
I also want you to notice the button just to the right of the one I have circled, the one with multiple XXX's on it. This button will create a corps HQ for this unit, assuming it isn't already a corps. If you clicked on it, it would create that HQ and then you would see you could click on it yet again to create an HQ above that at the army level, and so and so forth until you created an entire new army group chain of command which would be unattached to either theater. If you had a army HQ that was unattached to an army group, it would also directly allow you to create a new army group HQ's. Note that new HQ's will take some time to be properly staffed and organized, and until they have done that they won't benefit the troops under them much. You cannot create new theater HQ's, you can, however, request the AI to recalculate theater areas in the production screen. Note that in most cases, this will do nothing. The AI will tend to create new theaters if it has a new front (i.e you declare war with a neighbor and thus have a new hostile border). It will sometimes eliminate a theater if a border is no longer hostile (i.e once you conquer Poland, if you ask the AI to redraw the theatre areas, it may consolidate all of Germany under one theater).
In any case, ff we click on the button that's circled (same position as the detach button, one becomes the other as appropriate), we get a menu where you can select which HQ to add it to. Selected the HQ as appropriate, and viola! your division has rejoined the command structure. Note that the Hq's listed in the dialog are sorted by how far they are from your selected division, with the closest HQ's being near the top.