The Mobile Infantry is an elite planetary fighting force of the Terran Federation. This Branch of the Terran Federation Army is the main ground combat force with other active ground units being K-9 Corps used for Recon.
Heinlein depicts the Mobile Infantry as a heavily armored but highly mobile force which virtually dominates the battlefield and greatly utilizes shock effect — much in the way of modern armored units. Likewise, they are utilized in the same missions (raids, screens, interdiction, etc.) as modern cavalry. In effect, the role depicted in the novel, relying on mobility, shock effect, and firepower, greatly follows the modern US Army Armor branch, with components running the gamut from light cavalry to heavy tanks. In addition, elite airborne infantry is a chief inspiration for Heinlein's troopers (both parachute and Air Assault).
A Mobile Infantryman is equipped with a Powered armor suit as well as an arsenal of weapons, some built into the suit. These weapons range from conventional firearms to micro-nuclear warheads. The armor is strong enough to resist any conventional weapon less than high explosives or anti-tank rounds. The servo-motors give the wearer enough strength to crush a tank or tear through a concrete wall, but are dexterous enough to allow them to pick up an egg without breaking it. The armor is also equipped with jump jets and rockets, allowing the trooper to move rapidly about the field and cover entire square miles of terrain. One expression used in the book as a compliment to good soldiers is that they are "on the bounce". According to Rico, when one is suited up in the Powered armor, they appear like a "steel gorilla".
Usually, a Cap trooper of the Mobile Infantry in platoons would drop from individual capsules launched from a troop transports in orbit. The capsule is destroyed during descent. The Mobile Infantry platoons will then execute their missions. These missions range from hit-and-run strikes to search and rescue missions. The Mobile Infantry will usually deploy about one trooper every half-mile. After the mission is complete, the Transport will launch a retrieval beacon for the platoon to follow to the retrieval area. Usually the beacon will play music such as, in the case of the Starship Rodger Young, "The Ballad of Rodger Young." Once the retrieval boat lands, the platoon boards and returns to the transport.
The Mobile Infantry prides itself in its esprit de corps. This philosophy includes never leaving a wounded soldier for dead; instead they pick up their wounded and make sure they make it back alive. Also, all Mobile Infantry soldiers are combat soldiers; there are no dedicated support staff. This means that when a unit launches a mission, all soldiers do so.
The training of the Mobile Infantry is depicted as extremely strenuous, with a few dying before completion. The book's narrator, Juan Rico, describes how out of his entire 2000 man training unit, fewer than 200 make it all the way through.
- First Mobile Infantry Division "Polaris"
- First Regiment
- Third Regiment "Pampered Pets"
- First Battalion* (CO. Major Xera)
- A Company, onboard Normandy Beach
- B Company, onboard Normandy Beach
- C Company "Warren's Wolverines" (CO. Lieutenant Warren), onboard Tours
- First Platoon (CO. Lieutenant Bayonne)
- Second Platoon (CO. Lieutenant Sukarno)
- Third Platoon (CO. Lieutenant N'gam)
- D Company "Blackie's Blackguards" (CO. Captain Blackstone), onboard Tours
- First Platoon (CO. Quick Silva)
- Second Platoon (CO. Lieutenant Khoroshen)
- Third Platoon (CO. Rusty Graham)
- Second Battalion
- Third Battalion*
- First Battalion* (CO. Major Xera)
- Fifth Regiment
- Second Mobile Infantry Division
- Third Mobile Infantry Division
- Fifth Mobile Infantry Division
- "Chang's Cherubs" (platoon)
- Unit unknown
- Head Hunters
- King's Kodiak Bears
- Note: "*" mark refers units mentioned in the 1976 board wargame Starship Troopers.
- Camp Arthur Currie (Battalion commander: Ian Frankel)
- H Company (actually a platoon, CO: Charles Zim)
- Camp San Martin
- Camp Sergeant Spooky Smith
- Camp of unknown name, located in Siberia