A three-dimensional miniature or life-size scene in which figures, stuffed wildlife, or other objects are arranged in a naturalistic setting against a painted background.
In other words, build what ya want, TM...
It doesn't have to be military, no... According to Shep Pines, my personal diorama hee-ro, "A diorama, in the modern sense, has come to mean any scene exeecuted in three dimensions, whether it is enclosed in a box or not."...
IMNSHO, the best dioramas do two things:
1: They show the subject "doing it's thing".
2: They tell a story (and a side-story sometimes)...
The story should be obvious and immediately apparent to the viewer... The viewer shouldn't need the diorama action (or in-action) explained to him/her... If you include a narrative, it should be no longer than 2 or three lines...
If it takes a paragraph (or more) to describe what's happening, you've got a weak story... Any of the "Well, maybe this guy's doing this thing, or just finished doing that thing" stuff from a couple of viewers will result in confusion and generally indicates a poorly thought-out storyline...
Telling the viewer what's happening, what the story is, should be done in a single word or three, if any narrative is needed... An example of Shep's wording went like this: "A diorama of a Sherman tank, with a dead crewman hangin' out the hatch with the tank commander desparately trying to get his .50 cal working, but to no avail, could be titled with something like "Jammed!... Drama and action...
A couple of dismounted GIs at a crossroads (with a sign nearby reading "Malmedy-8Km", with their M8 Greyhounds pointing in different directions, and holding maps coud be titled, "Lost"...
With the M8s pointing in different directions, and one TC looking down at his map while the other points down a road, we know who's doing the talking, and who's doing the listening, and the story is readily apparent to the viewer... It's December, 1944, the Battle of the Bulge, just prior to the "Malmedy Massacre", and these two M8 crews are about to have a really bad day...
A '75 Vette at a gas station dio with a sign reading "NO GAS TODAY" hastily painted and set near the pumps, with the "Vetteman"
and Staion-owner in a shouting match tells us that it's 1976 or so, and the Arab Oil Embargo has hit home to the 'Vetteman...
So just build what ya want, and feel free to brain-storm about it...