The Catholic Church permits military service and affirms the moral legitimacy of just war, while also recognizing the right to conscientious objection. The just war tradition, developed from St. Augustine through St. Thomas Aquinas and codified in modern Catholic social teaching, sets strict criteria: just cause, right intention, legitimate authority, last resort, proportional means, and reasonable prospect of success. Failure to meet these criteria makes a war unjust and participation in it potentially sinful.

The Catechism (§§2302–2317) treats war and peace extensively. The Church's social teaching since the 20th century has increasingly emphasized peacemaking, diplomacy, and arms reduction, while not adopting absolute pacifism. Pope Francis and his predecessors have condemned nuclear weapons and called for disarmament. The Catechism (§2311) also states: 'Those who, for reasons of conscience, refuse to bear arms... are nonetheless obliged to serve the human community in some other way.' Military chaplains serve Catholic soldiers in armed forces worldwide.