The morality of war
The following quotation is taken from Fighting for the Confederacy, a personal memoir of the War for Southern Independence by General Edward Porter Alexander, who was a 26-year-old captain of engineers in 1861 (the time of this anecdote).
... I saw a very fine looking sergeant major come out of the woods on the left with a small man in citizen's dress and take him before the colonel at the head of the regiment. This turned out to be Col. Cash ...
At that time the colonel was a tall, stalwart fellow, apparently 35 or 40, red headed, red faced, light grey eyed, strong-featured &, as I approached him that afternoon, his face was as angry looking as a storm-cloud, & he had drawn his revolver & was trying to shoot the little citizen who was dodging behind the big sergeant major as Cash turned his horse about & tried to get at him, poking at him with the pistol & swearing with a fluency which would have been creditable to a wagonmaster. “You infernal s. of a b.! You came to see the fun did you? God damn your dirty soul I'll show you,” & he spurred his horse to get around the sergeant major.
“What's the matter, Colonel,” said I. “What are you trying to shoot that man for?”
“He's a member of Congress, God damn him,” said the colonel. “Came out here to see the fun! Came to see us whipped & killed! God damn him! If it were not for such as he there would be no war. They've made it & then come to gloat over it! God damn him. I'll show him,” & again he tried to get at the poor little fellow who was evidently scared almost into a fit.
“But Colonel,” I said, “you must not shoot a prisoner. Never shoot an unarmed man.”
I spoke authoritatively, for Beauregard had published an order that all officers of his staff spoke with his authority, & Cash recognized it & made no kick.
The young Alexander's prim correctness is rather amusing, and perhaps he was right to disapprove of Col. Cash, who may not have been an admirable fellow. But in one respect the choleric colonel was way ahead of his time: in recognizing that war is the greatest of all crimes, mass murder beyond the dreams of any serial killer, and politicians are the criminals responsible for it.
If anyone deserves killing in war, it's the politicians who authorized it and who could stop it at any time. The usual convention that politicians are to be treated with respect and not made military targets derives, of course, from the politicians themselves, who agree to make each other immune from the bloodbath they have created. It's not clear to me why non-politicians should respect this convention.
Justification for war
Can there be any justification for making war? Yes, surely. If an army invades your country and begins to kill people indiscriminately, without provocation, then I think you have to regard that army as an organization of murderers, and return fire in self-defence.
But this is a rather extreme and easy case. In general the case for war should be examined very sceptically, given the vast amount of death, suffering, and damage it causes.
Furthermore, the use of conscription in modern wars is very hard for a libertarian to accept. We might reasonably say to the politicians: if you want a war, fight it yourselves! Or at least pay volunteers to do it for you.