What is the Cost of War?

Paul Krugman's editorial in the New York Times makes the observation "that modern war is very, very expensive. For example, by any estimate the eventual costs (including things like veterans’ care) of the Iraq war will end up being well over $1 trillion, that is, many times Iraq’s entire G.D.P." One trillion is a gross underestimate. Which brings me to a question. What is the cost of war?  Except for World War II's positive effect for the American economy, there is no other instance in the past century where a war was beneficial. Vietnam caused the decade long recession of the 1970's and W's unilateral invasion of Iraq had much to do with the Great Recession.  Wars which do not develop future markets are failed investments that an economy absorbs with great pain.