In 'Starship Troopers', what branch of the military does the protagonist join?

In the 1997 film “Starship Troopers,” protagonist Johnny Rico joins the Mobile Infantry.

Rico initially enlists partly to remain close to Carmen Ibanez, who is training to become a starship pilot. The Mobile Infantry is the Federation’s ground-combat branch, deployed against the Arachnids, the insectoid alien enemy commonly called “bugs.” Rico undergoes harsh basic training under Sergeant Zim and eventually becomes a squad leader.

The film’s military structure can cause confusion because other characters serve in different branches. Carmen belongs to the fleet and becomes a pilot, while Carl Jenkins joins military intelligence. The Mobile Infantry also differs from the heavily powered-armored troops of Robert A. Heinlein’s novel: the film largely presents Rico’s soldiers as conventional infantry using firearms, dropships, and explosive weapons.