Earlier quoted context omitted.
A standing army creates a ton of problems, but what happens if nobody has a standing army? Seems like someone has to play cop and eat the cost (both social and economic) of doing so.
Like many social problems, this is actually a coordination problem, and can be easily boiled down to a tragedy of the commons situation. Everyone would prefer to not have an army, because most of the time you don't need them but they're still expensive to maintain and when you do need them they're even more expensive. But if nobody has an army then the bad actors can form an army and take over everybody else. So ever…
The closest we've ever come to seeing this happen in real time was probably with German rearmament in the 20s and 30s, and they were telegraphing for decades what they were doing.
It does however raise the other problem - if people are willing to stand by and ignore obviously bad actors skirting closer and closer to the line, you'll have at some point a fairly big problem, and it depends on how long you waited.
That is the bigger problem of global demilitarization - the fact that non-action in the face of a violation usually plays better to a domestic audience than actually doing something. (This is a pattern that has and will play out repeatedly. That's the actual question that needs answering)