by Patrick Appel
1. The immediate costs of war are clearly awful. Most wars lead to massive loss of life and wealth on at least one side. If you use a standard value of life of $5M, every 200,000 deaths is equivalent to a trillion dollars of damage.
2. The long-run benefits of war are highly uncertain. Some wars – most obviously the Napoleonic Wars and World War II – at least arguably deserve credit for decades of subsequent peace. But many other wars – like the French Revolution and World War I – just sowed the seeds for new and greater horrors. You could say, "Fine, let's only fight wars with big long-run benefits." In practice, however, it's very difficult to predict a war's long-run consequences…
3. For a war to be morally justified, its long-run benefits have to be substantially larger than its short-run costs. I call this "the principle of mild deontology." Almost everyone thinks it's wrong to murder a random person and use his organs to save the lives of five other people. For a war to be morally justified, then, its (innocent lives saved/innocent lives lost) ratio would have to exceed 5:1. (I personally think that a much higher ratio is morally required, but I don't need that assumption to make my case).