Ok well quite a few hundred reviewers have read the book and consider it virulently militaristic, and some go further considering it jingoistic, and fascist. I don't know enough about Heinlein, but I kinda rather doubt he was advocating fascism. But it would not surprise me if he thought civilian control of the military was backwards, because I've had this conversation with members of the military. And some of them have overtly expressed people who have not picked up a weapon and used it to defend the country (I guess by shooting and killing, but perhaps merely by taking the risk to their own life) should not have a right to vote. And that is directly expressed in Starship Troopers as a matter of public policy. If you don't serve, you aren't a citizen, you cannot vote, and you cannot be a member of the government.
This may not have been what Heinlein intended to convey. But I tend not to find an author's advocacy relevant when reading fiction. I see the author as giving me rope to hang myself if I choose, rather than considering that they advocate anything at all. As I read it, militarism so permeated that society that the only possible contrarian point of view would come from the reader. And I thought that was the whole point of the book. It doesn't matter whether that's Heinlein's intent.
If it's true Heinlein just didn't convey the vast majority of Federal Service was non-military, and yet all we have to go on is the exclusively militaristic bent as written, that deficiency might make the book all the better just to have the ensuing ideological battles and discussions that resulted. And this book generated quite a lot of them at the time, and I see it as overwhelmingly beneficial.
A likely metaphor to the non-citizens of Starship Troopers are those who of their own volition do not enter into military service, do not vote, do not participate in government - exist in large numbers in the U.S. Most Americans now do not vote, they're simply along for the ride, and in effect aren't citizens in any meaningful way (other than perhaps they can't be deported - but in Starship Troopers it was a world government so there's no concept of deportation anyway).