What is the theme in Starship Troopers by Robert A. Heinlein?

Citizenship is the main theme. Full citizenship in the future age in which Starship Troopers is set – meaning the right and obligation to vote and hold public office – is strictly limited to those who have honorably completed a term of service in the armed forces of the Terran Federation. This criterion develops gradually out of a period of social turmoil in which young people are raised, believing they have inalienable rights but no responsibilities.