Planetary Defences -> The Ring/The Sphere

Imperial Infantry -> The Shield/Shield Troopers/The Guard/Guardsmen/Defenders

They could have lines like, “The Shield must stay strong,” “Defenders of Earth/Planet X,” etc.

Storm Troopers/Shock Troopers -> Attackers/Infantry/Attack Force

Militia -> Militia/Armed Civilians/Civilian Defenders

I would argue that it's first and foremost important to pinpoint the role of your units with a name as simply and clearly as possible. Very pragmatically. For complex units this may require writing actual phrases. From there you can use flavor language to simplify further, and/or fit the thematic of the faction and game.

For example, from Command & Conquer: Red Alert 2, the basic Infantry unit:

Infantry -> G.I. (for the Allied faction)

Infantry -> Recruit (for the Soviet faction)

As for using “flavor” to simplify, a special unique unit from the same game:

fast mechanical drone that can kill infantry in one hit and attach itself to vehicles to destroy them from seemingly within -> Terror Drone

I think Infantry and Space Infantry are perfect as far as functionality goes. I feel the word infantry lacks some flavor, though. Hmmmm, let's see.

Soldier -> Space Soldier

Marine -> Space Marine

G.I. -> Space G.I.

Gunslinger -> Spaceslinger(?)

Droid -> Space Droid

Android -> Space Droid

Recruit -> Space Recruit

Cadet -> Space Cadet

Ewok -> Space Ewok (okay, not really :) )

Goblin -> Space Goblin (if you want to bring in some fantasy?)

Dragon -> Space Dragon

etc.

Just some brainstorming. You can easily replace the word infantry with anything like that now, and the roles are still pretty clear, which is great. Because now the functionality is clear, and you can focus only on what flavor you want for your game—serious, silly, cute, scary, drunk ... :cool:

I would also suggest using the word "Unit".

Also some more suggestions for names:

Spacebourne Ground Strike Unit - "Spacebourne" signifies that it is by default garrisoned on a spaceship and whereever their spaceship goes, they will go also. "Ground Strike" signifies that this unit is specialized on offensive actions on the ground of a planet.

Spacebourne Ground Defense Unit - Like the above, but is specialized on defense. They are basically these units that one can easily moved within the empire. These Spacebourne Defense Units are basiacally the first units to respond - they are those who come very fast to stall the enemy long enough for the cavalry to arrive. Strike unit would be that cavalry.

Spacebourne Space Strike Unit - Space Strike Units are specialized in neutralizing spaceships. They can assault a ship and board and/or destroy it from within. They also man the fighter jets.

Spacebourne Space Defence Unit - Units who are specialized in protecting spaceships. They are those who will fight off any boarders and attackers.

Groundbourne Ground Strike Unit - Only thing that is different from Spacebourne Ground Strike Unit is that those units are permanently stationed on the ground. They have weaker training, but they have faster recruitment speed, since they can always draft more amongst the locals.

Groundbourne Ground Defence Unit - self apparent from the naming convention.

Groundbourne Space Strike Unit - Stationed on planet, but are good at attacking fleets on the orbit. They usually support the Spacebourne Space Strike Units.

Groundbourne Space Defence Unit - Stationed on planet. They defend obviously. They shoot missiles at enemies. They man the anti-air turrets on the ground. They deply tactical energy shields where neccesary. They are basically anti-air, plus anti-space.