read spider-robinson's essay : 'rah rah R.A.H !'. imho, anyone trying to pigeonhole robert-heinlein into any of pejoratives e.g. fascist, racist, militarist etc. etc. has either an axe to grind, or has serious reading comprehension problems.
Actually, if you take a look at the man's literary career I think he is just experimenting with the "what is best ruling regime?" "Starship troopers" is a possible answer, so is the "moon is a harsh mistress" and I even see the "Stranger in a Strange Land" in this light.
My two cents.
Starship Troopers is sometimes viewed as his "parting shot" to the left wing, though it was written several years after he ceased to consider himself a Democrat.
While it probably wasn't as militaristic as it appears at a casual glance (he's elaborated on the ideas elsewhere in ways that further "softens" it), it did come at a time where his political views were evolving towards a more conservative and more authoritarian ideal (though that soon changed again).
Militarism is intrinsically fascist. It's all about banding together, being stronger when pulling in a single direction, under the leadership of the state, and not brooking any dissent. It's intrinsically a dangerous social setup only suited to extremes of survival. I'm not a fan of fans of military qua military.
Pigeonhole as in "he was just/only a militarist" no.
But he was clearly a militarist.