Symbolism, Imagery, Allegory
Ladies and gentlemen, let's talk about Marxism.
In a really, really concise nutshell, Marxism argues that society, instead of being organized according to who has the most money (or "capital"), should be organized around who has the "means of production," or who actually works and makes the stuff that helps society function. No 1% here.
Now, there's way, way more to Marxism than this, but for the sake of this book, that's pretty much all you need to know.
So what's the deal with Marxism in Johnny Got His Gun?
Remember those "little guys" who keep popping up all over the novel? Whoever they are, it seems as if they're the ones always being sent to war to do the dirty work of... presumably the big guys, who are also never completely identified.
Part of the reason why Trumbo uses these vague terms instead of having Joe recite an all-out Communist Manifesto (which he sort of does, anyway) is that he's trying to show the general logic behind the idea instead of going all political and trying to come up with an agenda that the reader will automatically judge. There may be political consequences to the stuff Joe is saying, but the stuff he's saying is based on something deeper than just politics.
The "little guys" first appear when Joe obsesses about war at the end of Part I (10.10, 10.18, 10.25), but it's not totally clear who he's referring to—beyond, you know, the everyday people who get drafted into wars. We don't get any sense that there might be Marxist undertones in this idea until the end of the novel, in passages like this (the "we" is the little guys):
[W]e we we are the world we are what makes it go round we make bread and cloth and guns we are the hub of the wheel and the spokes and the wheel itself without us you would be hungry naked worms and we will not die. (20.30)
Whoa. Where did that come from?
Joe thinks that it's the common people like him who have the power to prevent future wars. Joe thinks that governments—and the rich, powerful in charge of industries that can profit from war—will continue to get into wars because 1) it benefits them economically, and 2) they don't have to do the fighting themselves, so there's nothing really at risk for them. If all the "little people" wised up and just said no, says Joe, there wouldn't be any wars.