Patrick Renfield began this discussion with a contradiction: 'War is bad', and it has been an integral part of humanity since the dawn of time. A cursive reading of history quickly reveals that the history of human civilization is comprised of wars. Our civilization records wars as the events which civilization turns and progresses. It's easy to dismiss this fact by talking about the militarization of history, but that's really a form of intellectual denial.
The reality is war is an integral part of civilization and economic development. This case has been made most convincingly in recent years by Prof Niall Ferguson's works like 'The Ascent of Money'.
Ancient wars were wars of annihilation. For whatever reasons, one tribe (or culture) attacked another. In the ensuing war, one or the other of the tribes was destroyed. Their cities were burnt to the ground and their people killed, or scattered, or enslaved (which may be the most complete form of assimilation.) The valuables of the defeated tribe were carried back to the conquering city to make the king or general richer and more popular.
As history marched on, this pattern proved wasteful. Fewer soldiers were killed relative to the overall population and the number in the opposing forces; and far more captives were taken. The result is fewer societies were annihilated if they went to war.
Civilization has progressed to the point that the value of a population is no longer only measured in what can be taken from their dead bodies. In fact, civilization has progressed to the point that we see people as economic units: sources of taxes, work, and innovation.
But war still has a role in society. It is the most efficient means to bring societies together to a common purpose. War is a means to drive innovation and technical development. It's far easier to get funding for new weapons than new cars, for example. And, war is a means for a government to ease unemployment, inflation, and nearly all the other evils of modern economic theory.
As a result, a kind of war has appeared. Instead of a war that can be won with definitive goals within a time frame, we have invented sustainable war. One form of sustainable war is the peacekeeping force. Vietnam, Iraq, and Afghanistan are the most prominent examples, but the concept is as old as the Roman Empire. (Read about Trajan and why he didn't follow through on his victory.)
Sustainable wars have very few casualties, and almost never a real victory for anyone, but they do allow billions to be spent on technology; and a great deal of energy spent in support of troops.
As a civilization we have changed the morality of war from one of survival to one of supporting the quality of life within our society. The changes were not instantaneous. It a couple thousand years for all the concepts to be digested and understood. There is very little reason to doubt that any subsequent society will not build on these concepts.