I figure in 5E, the easiest way to do this is just to stat military units as high-Challenge creatures. A troop of 25 soldiers would be a Challenge 4 Huge creature with, like, 125 HP and four attacks per round. Attacks that affect an area deal double damage to the unit if they include 2 spaces of the unit, or triple if if they include the troop’s entire space.
A battalion of 300 soldiers would be, like, a Challenge 20 Colossal Creature (do they have colossal in 5E?), with 1000 HP, multiple high-damage attacks, reactions, and legendary actions to represent different tactics. It might have a really long 'reach' to represent there being troops arrayed in defensive lines outside the main body, and the ability to make unlimited opportunity attacks.
Now, unit vs unit tactical combat is different than long term military strategy, with supply lines, scouting, sieges, etc. That sort of stuff would deserve its own book, since unless you're Sun-Tzu, you can't really condense the entirety of all military knowledge to a short chapter.