Hey,
As was said above, war is the continuation of politics by other means.
It is the direct result of two reasoning groups of people failing to agree. In simplistic terms, think of two tribes on either side of a wheat field. Both feel they have a right to the wheat, and both MAY be correct. For a time they can split the field and thirve. But there comes a time when both tribes have grown, and now they require more of the wheat field to sustain themselves. Since both tribes face the same problem, and there is only one field, they are at an impass. Both need the wheat; both have rational reasons why the wheat should be theirs; but the reality is that there is only so much wheat (land) and both sides can't afford to share. Result: violence. As humans have evolved, the equation may have changed values (substitute gold or land or religious differences for wheat), but the result is always the same.
The most expedient and long lasting way to resolve these types of conflicts is to remove the opposing group, permanently.
A phrase I coined while taking military history in college was "Anyone who says violence doesn't solve anything has never been killed in a dispute."
It really boils down to space. There is only so much of it, and some parts are better than others. Every group feels entitled to their own "space" but if its all taken already, they have to fight the current tenants for it. This could be for food, prosperity, freedom to pursue an ideal, etc...
Obviously this is boiled down to almost inconsequential levels on this forum, and this is a subject which has filled volumes of text over the years, but that is my super-simplified take on war.
Reaper
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"We're in business, definitely!"
Mike, Saving Private Ryan