I think that war is overrated. When people kill others in the name of war or in defense of their country, it isn't considered murder. If I walked out of my house and shot someone because they were challenging my freedoms, I would be arrested and imprisoned. It just doesn't seem consistent that a soldier can do the same to a foreigner and be rewarded for it.
That being said, I think that war creates a much stronger divide between nations: even between nations that we aren't at war with. It perpetuates an "Us versus Them" mentality. More disturbingly, I think that war dehumanizes people from warring nations. That's why killing under war's guise isn't viewed as murder; the people that are killed don't seem to be people at all. In reality, these people were born, had lives, friends, and family.
In addition, I think that war prevents us from seeing the somewhat hypocritical aspect of its polarizing effect; the warring nations' armies don't see our armies as human either. When they fight and kill the soldiers from our country, they don't see them as having lives, friends, family, etc. In their eyes, they're not murdering anyone; they're fighting to protect their ideals. So in their eyes, they're justified in the exact same way that we're justified in our own eyes. Obviously, it doesn't make sense that both sides can be the good guys while the opposite side is inhuman (and to assume that one's own side is the right one is pretty presumptuous).
In the end, the only real positive effect that war seems to have is that it gives the 'victorious' nation(s) a vast upper hand in end negotiations. However, this perpetuates the idea that the nations with the greatest military power are the just ones. And, in practice, the nations that have won wars attempt to make this idea true through their history. When we go to war, we assume that we will be the winners because we've always been taught that the noble ones win (and of course we see ourselves as the noble ones). But again, the other warring nation is thinking the same thing. It seems that when war is involved, every nation is an angel and a devil at the same time.