Just-war theory has a long and distinguished history that stretches back to the Christian theologians of medieval Europe. Yet principles of just war must develop alongside social norms, standards of military practice and technology, and civilian-military relationships. Since World War II, and especially since American involvement in Vietnam, military ethics has developed into an academic cottage industry. As commonly taught to undergraduates and military practitioners, contemporary just-war theory seeks to ensure the political sovereignty and territorial integrity of nation-states. The theory insists that the only just wars are defensive ones and forbids wars of national aggrandizement. On this view, because of the right to collective self-determination, wars must not seek to remake the world order, as that would undermine state sovereignty.In recent decades, however, cosmopolitan philosophers have challenged various aspects of the traditional edifice in an attempt to use just-war theory to enhance the protection of human rights around the world. Scholars have argued for greater scope for humanitarian intervention to protect individuals against their own government, for principles of justice after war to ensure that all states are legitimate, and most radically, for the responsibility of ordinary combatants to assess for themselves the justice of their military's cause. On this last argument, because combatants whose cause is just have done nothing to lose their immunity from harm, attacking them is unjust, and combatants whose cause is unjust cannot fight with discrimination.This publication surveys these recent developments, and it finds that they provide a radical challenge to both the theory and the practice of contemporary warfare. Of particular importance is its insistence on the need to strengthen international institutions, so as to provide combatants with an impartial perspective on their side's cause, and to strengthen military ethics education; and its suggestion that policies on dishonorable discharge be rethought. However, this monograph also challenges certain aspects of the new approach, suggesting important connections between military ethics and democratic theory and practice.