Kevin Drum puts his finger on it:
It is, often, not so much war itself that people long for, but the moral certainty that comes with it; thus the venom directed even toward those who are skeptical of war, let alone those who are resolutely opposed to it. It's not that the skeptics prevent the hawks from getting the war they want — they usually don't — but that they deny them the moral certainty they so desperately yearn for. And that cannot be tolerated.
Many people want to believe there is a clear demarcation of Right and Wrong but the uncertainties of our contingent world often spoil their attempts to sustain that belief. War, as Kevin says, clears the picture of all those confusing contingincies and gives, at least for the moment, a sense of moral certainty. Chris Hedges wrote similarly in his book, War Is A Force That Gives Us Meaning (highly recommended). War, for all its terrifying aspects, makes us feel alive and certain about our direction.
War, like a torrential downpour, clears the air of the smog of uncertainty and gives us, if even for a brief moment, a feeling that the path forward is clear. War, like a forest fire, clears out the underbrush choking the forest so that new trees can grow.
But floods and fires also drown and burn. Floods and fires are forces of nature, unleased in nature's own time for nature's own purpose. War, if it is to be seen as a force of nature, should never be seen as something desirable. It is, at best, a consequence of the natural process of life and, like death, something that comes in its own time and its own way.
It makes no more sense to sound the drumbeats for War, to see War as a good thing, than it would be to cheer on the burning of a forest or the flooding of a coastal village.
Those who think they can be the Masters of War are creating a false idol. And, like all idolators, they will eventually burn in the fire of their own arrogance or drown in the flood of their own hubris.
We will never be the masters of War. The best we can do is learn how not to be its slaves.