The opposition to America’s entry into both World Wars was largely led by conservatives. Senator Robert A. Taft, the standard-bearer of postwar conservatism, opposed war unless the United States itself was attacked. Even Bismarck, after he had fought and won the three wars he needed to unify Germany, was staunchly antiwar. He once described preventive war, like the one America is being pressured to wage on Iran, as “committing suicide for fear of being killed.”
Conservatives’ detestation of war has no “touchy-feely” origins. It springs from conservatism’s roots, its most fundamental beliefs and objectives. Conservatism seeks above all social and cultural continuity, and nothing endangers that more than war.
http://www.theamericanconservative.com/articles/why-conservatives-hate-war/
