The Catechism of the Catholic Church states:
“Every act of war directed to the indiscriminate destruction of whole cities or vast areas with their inhabitants is a crime against God and man, which merits firm and unequivocal condemnation.” A danger of modern warfare is that it provides the opportunity to those who possess modern scientific weapons—especially atomic, biological, or chemical weapons—to commit such crimes. (CCC 2314; cf. Gaudium et Spes 80)
Under the Catholic understanding of just war, not only must the cause of war be just, but the acts of war used in combat must also be just. The use of weapons to destroy civilian populations, such as the atomic bombs dropped on Hiroshima and Nagasaki, is an unjust act of war. Terrorizing civilian populations to force surrender, which was part of the U.S. strategy in dropping the atomic bombs, is also an unjust act of war. It is therefore problematic for a Catholic to try to justify the U.S. decision.