Faced with the destructive power of modern weapons, it is very difficult to speak, as was done in past centuries, of the possibility of a ‘just war.’ As early as 1963, Pope John XXIII, in ‘Pacem in Terris,’ wrote that in the atomic age it becomes almost impossible to think that war can be considered an instrument of justice. In this same spirit stands Pope Leo XIV, who is making peace one of the central themes of his pontificate.