I know this could provoke hard feelings from people who work in the army or have family members there, but I'm putting it completely theoretical. Let's see...
To be opposed to unlawful behaviour is OK, because breaking of laws is anti-social behaviour and bad for all of us. We have police to stop that kind of behaviour. And to be a police officer is a honourable job - your calling is to protect the society as a whole.
War is unlawful in more than one element - it's destruction of property, murder, grand theft and a lot of other offences rolled into one. To stop this we need some kind of heavy armed police.
But army is not what we need. It's composed of people who consciously decided their job should be killing other men. If they wanted to protect their society they would become policemen. This way they choose to train killing and be ready to go kill at the moment their leadership demands it. They are career criminals waiting for the licence to kill.
To prove the point, even the leadership of the police and army is separate - so they are not two arms of one body meant to protect the society.