The issue is that while someone is building an army, the rest of the world might stand by and do nothing in hopes that the army won't be used, via appeasement and negotiation, particularly if the world's culture is strongly set against having armies. At least until the aggressor starts conquering. That's how it tends to play out historically, so countries and kingdoms go to the trouble and expense of having standing armies, or being able to call them up quickly enough. Because there's always potentially someone who gains power that wants more, and plenty of people willing to follow.

> particularly if the world's culture is strongly set against having armies.

If the world's culture is strongly set against having armies, and one country effectively threatens them by breaking that taboo, then I would at least expect the world to instantly cease all trade and transport links with the arming country.

That may not be enough on its own to force the taboo-breaking country to disarm, but I don't see any contradiction in a world culture which is against having armies while also ruthlessly dedicated to temporarily re-arming and exterminating the first country that breaks the taboo.

There are hard coordination and rule of law / presumption of innocence questions to solve there, but I don't think we can necessarily assume that this world culture will stand idly by while their enemy works to undo the very thing that defines the culture.