The military of the UPMW has luckily never had to fight any real, all-out wars with opposing civilizations since its founding. Its main role is patrolling space, sometimes having to protect ships from pirates or catch smugglers. In a way, they largely act as an interstellar police force, as opposed to actual police which usually operate only on the planetary scale. The military even has its own investigatory division separate from the IPC for more international affairs.
Sometimes, the military force does need to “bring out the big guns,” usually to planets with unsavory activities like revolutions or local wars going on. Not uncommonly, these planets end up splintering off from the UPMW. Sometimes, non-UPMW planets will even call on the military for help, and depending on the situation they may comply. A relatively recent example of this was when Lazarus called on the military’s help stopping what they viewed as a cruel warlord.
While there are some smaller interstellar human governments that might like to conquer UPMW planets for one reason or another, they know they would never win, and would just end up wiping themselves out. In this way, the large resources of the military act as a deterrent.
In terms of structure, the military mimics the high council by having no single leader, instead having a cabinet of the highest-ranking commanders. The number of members in this cabinet varies, but usually doesn’t exceed a dozen.
Below commanders are the generals ranked 1-5, who usually are responsible for some sector of space. Then a lower ranking general will be responsible for a smaller subset of that space, etc. However, multiple generals of the same rank can be in charge of the same area.
Below generals are colonels, who are in charge of individual fleets of ships. Individual ships will then have a captain leading it. Squads, whose members are usually all kept on the same ship, are then led by lieutenants, being either a ground squad or a squad that flies smaller fighter ships together. Other members of the squad will have a rank that varies by experience, specialty, and performance.
An average ground squad may look something like this:
Leader (Lieutenant)
Assault (Typically at least 3)
Medical specialist (Typically 2, in case one is injured or killed)
Tech specialist/cyber warfare
Explosives
Heavy weapons operator
Sniper
In addition, the squad may have other specialists unique to them or even specific tasks. For ship combat, squads are different and less rigid. They’re usually composed of one pilot, multiple gunners, tech people, etc.
The military also employs specialized agents and covert squads for various purposes.