Reinhold Neibuhr rose to prominence in 1930s and 1940s for his vociferous opposition to both Nazism and to isolationism as an American response to that threat. He rejected both pacifism and the legalism of the just war tradition. In their place, he developed a pragmatic and realist approach to the ethics of force in defense of the liberal-democratic state. With the revival of the just war approach in the Vietnam era, Neibuhr's mode of ethical evaluation of the state use of force came to be criticized for its flexibility and lack of firm restraints. This work assesses, from the perspective of political theory, Neibuhr's ethical approach to war. It highlights the merits and the pitfalls of his approach and contrasts it to traditional bellum justum.