Uh… I mean there’s Halo Wars, which lets you play as Covenant leaders in certain modes (in 2 there’s a DLC storyline where you play as the aliens again, but not against the humans). The UNSC is basically the US Army In Space.

Aside from that I’m not sure. There’s probably plenty of mods for other games though. Like GMod, there are tons of NPCs and such in the steam workshop, plenty of US military characters to choose from. You can then load into a map, set up a spawner (or spawn them in yourself), and start blasting. I did actually do something like this back when the George Floyd protests were going on: I went in the game, spawned in a bunch of cop NPCs and just started blasting and mowing them down.

It actually did feel pretty good, tbh.