Sometimes war is absolutely necessary, as when a hostile nation attacks a peaceful one for financial or territorial gain. The peaceful nation's two choices are: 1. Submit and be robbed / killed / dominated by the hostile attacking nation, or 2. Go to war. History has shown that hostile nations generally do not even treat their own citizens well, to say nothing of the nations they conquer. Therefore, any people conquered and absorbed by the hostile nation will likely be treated as subhuman. So realistically, the peaceful nation has to go to war. It's either that or spend the rest of your life on your knees as a slave, assuming you're even permitted to live.
Now as for the people who initiate wars, someone far more experienced and eloquent than myself has already voiced their thoughts on this subject. Here's a speech by Smedley Butler, the most decorated Marine at the time of his death:
WAR is a racket. It always has been.
It is possibly the oldest, easily the most profitable, surely the most vicious. It is the only one international in scope. It is the only one in which the profits are reckoned in dollars and the losses in lives.
A racket is best described, I believe, as something that is not what it seems to the majority of the people. Only a small "inside" group knows what it is about. It is conducted for the benefit of the very few, at the expense of the very many. Out of war a few people make huge fortunes.
In the World War [I] a mere handful garnered the profits of the conflict. At least 21,000 new millionaires and billionaires were made in the United States during the World War. That many admitted their huge blood gains in their income tax returns. How many other war millionaires falsified their tax returns no one knows.
How many of these war millionaires shouldered a rifle? How many of them dug a trench? How many of them knew what it meant to go hungry in a rat-infested dug-out? How many of them spent sleepless, frightened nights, ducking shells and shrapnel and machine gun bullets? How many of them parried a bayonet thrust of an enemy? How many of them were wounded or killed in battle?
Out of war nations acquire additional territory, if they are victorious. They just take it. This newly acquired territory promptly is exploited by the few the same few who wrung dollars out of blood in the war. The general public shoulders the bill.
You should read the entire speech at: http://www.lexrex.com/enlightened/articles/warisaracket.htm