I myself am not an expert in the field, nor do I delight in the concept of human beings killing each other for any reason - that's the beatnik in me coming out I guess. But this passionately expressed viewpoint of my friend set me to thinking, and this is what I came up with: war is similar in many ways to a forest fire.
Forest fires are completely natural and in fact necessary for diverse and healthy growth. They clear out dead materials, enrich the soil - some seeds don't even become viable unless they are put through the heat of a full-fledged forest fire. As far as I can see from historical examples, war is similar to this in that it clears the bracken of apathetic and compliant thinking and forces society to consider active responses to threat, injustice, and inconstancy in their own framework of thinking. It also enriches a nation's sense of community: Canada was said to have become a nation during the First World War, as an example. Admist the panic and destruction of war, seeds of future minds that will rebuild the world are planted. Even though forest fires are often beyond human control, people who start them, either through criminal intent or negligence, are the guilty ones. Firefighters are the peacemakers who lead to the end of destruction and allow a time of peace for the ecosystem to rebuild itself and flourish.
Just because war is "natural", however, doesn't make it good, and this is where it deviates from the forest fire metaphor. Human nature is flawed, otherwise, we would be able to peacefully deal with challenges that result from the complex framework of world politics. War is a result of greed for power, resources, and the flawed belief that in some way one nation is more important or worthy than another. And an even scarier thought, war can be a result of boredom. North America, we have more than any other land mass in the world, yet we feel more depressed, disillusioned, and isolated on average than any other - and our "first world problems" leave us with little sense of greater purpose or goal. Our culture is on the decline. It's easy to see why some may want to stir people into a nationalistic crusade.
There have always been warmongers and peacemakers. The question perhaps isn't so much what's more "natural", but what is more reflective of what the world should be. Myself, never having lived through war, civil unrest, or bloody revolution, perhaps shouldn't be delving into such territory, but unless we think about these things in times of freedom, how will we be able to in times of oppression?