The very definition of justification is enforcing that this particular belief or opinion is right. While there ARE things such as evil, all parties believe they are doing the right things in accordance with their beliefs. If they are questioned in such a manner that could be threatening, then a war starts. It's the simple truth.
If there is a Christian, among others, who believe war is evil and there is no justification, read your book again - because there WERE wars - a lot of it, and God himself had influence over wars, all under justification.
There is a distinction that should be made, between wars of aggression, and wars of defense. If you are attacked, the only real choice is between capitulation, and resistance. A pacifist must not only be willing to sacrifice his own life for his ideal, but those of all of his loved ones.
Anyone who is intolerant of this, who values the lives of others more than his commitment to non-violence, is morally obligated to fight back. The defender never gets to choose non-violence; the choice always belongs to the attacker. If you choose not to fight, you can still be killed, raped, tortured, etc. And see others suffer the same.
So the question becomes whether any given war is justified. To which the answer is "Which side are you on?". The aggressor, the side which initiates force, is wrong-- but the side which defends itself is justified. This principle holds not just for wars, but criminal assaults, school yard bullies-- any and all encounters of force.