My intent is not to get too political here, but as I'm head-first into a ton of history at the moment, I've been finding the liberal peacenik question "what is war good for?" a little odd. Not that I'm a war mongering conservative asshole, mind you, but it is definitely a funny question ask. Especially because the answer is, well, it's good for quite a bit.
Let's just work within our own history, shall we?
First of all, war gave us our independence. We are no longer British citizens, at liberty to British taxes and governmental whims. Sure, we might have just replaced one set of jerks with another, but at least now, by law, we are represented in government.
Second, war eradicated one of the follies of human history: slavery. No war? Then, to be blunt, blacks (and other minorities, likely) would still be tilling our fields for zero pay and the occasional lashing.
Third, war eliminated two of the arguably most evil empires in the history of the world. Nazi Germany and Imperialist Japan. Hitler is dead and his empire is gone because of his opponents' willingness to go to war. The same can be said for Tojo and his ilk. The Jewish race is certainly happy we went to war, and so am I. Further irony is added due to the party affiliation of the US President that brought us into the war, but I digress.
There are many, many other examples of this throughout world history, but I'm not going to go into them all. Thanks to the US education system, you probably wouldn't be familiar with any of them, anyway.
My point here is not to suggest that war is a fine way to go about one's business, but it is not the all-encompassing "bad" way to get something accomplished. Sure, it sucks, but sometimes it is necessary... and often is the best course of action a people can take.
And, no, I'm not implying anything concerning our current global climate... I'm just making a point from a historical perspective (never mind a technological one). Basically, it takes completely ignorant people to suggest that war is good for nothing.
Seriously, where have they been?