In a presidential debate with a sitting vice president, a candidate from the other party distanced himself from the activist foreign policy of the previous administration in favor of a more restrained approach to the world. “I don’t think our troops ought to be used for what’s called nation-building,” he declared. The candidate also warned that America’s overwhelming power was both a blessing and a curse. As he explained, “Our nation stands alone right now in the world in terms of power, and that’s why we have to be humble.”...