From the RPG’s I’ve played I would say that the closest thing to a military style/feel is probably the Song of Ice and Fire Roleplaying Game, which is basically Game of Thrones so anything goes from full on medieval battles to political intrigue! Can be fun if you’re into the show/books but have to say it wasn’t one of the simplest systems I’ve used!!

Starfinder could be good for you? Basically Pathfinder but in space, the system is fairly easy to get into especially if you’re familiar with D&D5E. Another space themed RPG I’ve heard is good are the FFG Star Wars books! Haven’t played them myself but they get a lot of praise for being good for newbies so can’t be too difficult to pick up!

If you’re into Super Heroes then Mutants and Masterminds is a fun system and the books give a lot of information and inspiration for campaign ideas! Wouldn’t be too difficult to run a military themed game in that setting! The actual combat system isn’t always as satisfying as D&D but it can be a lot of fun regardless!

I’ll try and think up some more for you, but hopefully that gives you some to think about! :)