It’s not true that "only anti-American leftists" object to the indiscriminate killing of civilians. Whether you're talking about Hiroshima, Nanking, Nagasaki, Coventry, Hamburg, Guernica, Tokyo, New York City or anyplace else, the moral law acknowledged by Judeo-Christian civilization is the same and does not change. The deliberate shedding of innocent blood is an abomination.
"Any act of war aimed indiscriminately at the destruction of entire cities of extensive areas along with their population is a crime against God and man himself. It merits unequivocal and unhesitating condemnation. "
(Catechism of the Catholic Church)