It's a form of prisoner's dilemma: because the other guy may betray you (by launching a war), you have to be ready to deal with that, which means having a capable military establishment, because it isn't something you can throw together on the spur of the moment.

In practise, most militaries are set up to fight defensive wars (and to take part in peace enforcement). But some have leaderships which are aggressive on a political (Cold war Era Russia) or nationalist agenda (China and Modern Russia),or personal aggrandisment (Saddam Hussein's Iraq).