George Mason University economist Bryan Caplan makes the case for pacifism:
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….
While I admit that wars occasionally have good overall consequences, it's very difficult to identify these wars in advance. And unless you're willing to bite the bullet of involuntary organ donation, "good overall consequences" are insufficient to morally justify war. If the advocates of a war can't reasonably claim that they're saving five times as many innocent lives as they take, they're in the wrong.
I suspect that economists' main objection to pacifism is it actually increases the quantity of war by reducing the cost of aggression. As I've argued before, though, this is at best a half-truth:
Threats and bullying don't just move along the "demand for crossing you" curve. If your targets perceive your behavior as inappropriate, mean, or downright evil, it shifts their "demand for crossing you" out. Call it psychology, or just common sense: People who previously bore you no ill will now start looking for a chance to give you a taste of your own medicine.