War is the ultimate expression of tribalism, which, simply put, is “Us vs. Them”. We have many less radical ways of manifesting our tribal tendencies – family ties, team sports, religious affiliations, etc. – but when the division between ” us” and “them” becomes absolute, so that “they” are seen as utterly alien, and when this happens on a national scale, then conditions are ripe for war.
To be able to psychologically justify destruction on that scale, we have to convince ourselves that “they” are so unlike us that co-existence is impossible. We can then think of them as something other than human beings very much like us. Compassion doesn’t apply to them.