Since the dawn of civilization, human beings have been going to war with each other. This is not to say that humans weren't killing each other long before the emergence of agriculture, writing systems, animal husbandry, metal-working, and other "civilized" behaviors. As long as humans have existed, they've used whatever means they could to fight each other.
But armies and warfare are something that really only emerged around the 3rd millennium BCE. During this time, the conventions we would recognize as war — standing militaries, coordinated maneuvers, siege warfare, and controlling captured territory — began to appear. As time advanced, armies and warfare evolved to incorporate new technologies and other developments.
You could say that warfare is a yardstick with which the progress of civilizations can be measured. To put it another way, a civilization can be measured by examining the content and disposition of its armies. In the past century, human civilization has changed drastically, and those changes have been reflected in how we go to war.
By the middle of this century, this is likely to change even much, MUCH more! With the rapidly accelerating pace of technology and questions about the fate of human societies, these changes are likely to be drastic. In fact, it could become revolutionized to the point that our ancestors would not even recognize it as "war."
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