I'm not debating who the aggressor is or who started the war. It's not relevant to this situation.
It is extremely relevant to the situation.
The soldiers on one side die pointless deaths for their leader's vanity. If you tell them they'll die if they join it's a deterrent, because they can stay home and be safe.
The soldiers on the other side die protecting their home and their people. If you tell them they'll die if they join it's not a deterrent, because they'll die if nobody joins as well.