Because the military and war is like putting a society in a pressure cooker. Nothing good comes out of it.
We have been told in most stories that this is a time when people come together and stand up but this is really propaganda.
The reality is most people go a little bit crazy and paranoid in these situation. Understandably.
For example, friendly fires have always been very much under reported. Can you imagine what it lead too when an army is already a bit suspicious of each other?
Dude, it's kids. Their mind isn't set in stone, their opinions aren't very founded. Basic military service also includes ethical/political education. People are not getting packed into trenches yet, it's not prison. If anything, this is going to help tensions in society. They will share rooms, showers, dirt and sweat. We're all the same. Nothing has been shown more effective at tearing down prejudices than actual exposure and confrontation.