Isaac Asimov wrote in Foundation that, "Violence is the last refuge of the incompetent."
Robert A. Heinlein in Starship Troopers wrote, "Anyone who clings to the historically untrue — and thoroughly immoral — doctrine that "violence never solves anything" I would advise to conjure up the ghosts of Napoleon Bonaparte and of the Duke of Wellington and let them debate it. The ghost of Hitler could referee, and the jury might well be the Dodo, the Great Auk, and the Passenger Pigeon. Violence, naked force, has settled more issues in history than has any other factor, and the contrary opinion is wishful thinking at its worst. Breeds that forget this basic truth have always paid for it with their lives and freedoms."
Two of my favourite forces in SyFy (Douglas Adams, of The Hitchhiker's Guide to the Galaxy fame, sadly has little salient to say on the matter), each with opposing veiwpoints.
But, honestly, which is it? Is war nessicary? Heinlein makes a good point for it in Starship Troopers. The book's even on the required reading list of 4 of the 5 military academies in the US. Someone must think he has a point. But what about Asimov? His psychohistory is spot on in most places... so is he right? Are they both?
[Cue too many nights spent thinking about this question in darkened rooms with soulful music.]