The first part of August brings us to the anniversary of the events leading to the end of the war against Japan in 1945. By the summer of 1945 it seemed clear that Japan's power to wage aggressive war had been crushed. What remained was to force Japanese leaders to accept their defeat and surrender to allied (mostly American) forces. This was accomplished when an atomic bomb was dropped on Hiroshima and three days later on Nagasaki. These events always raise the question of the morality of these acts in the light of Catholic moral teaching.
Reading the word's of Jesus in Matthew's gospel you might get the impression that Christians are called to be pacifists.
"38 ‘You have heard that it was said, “An eye for an eye and a tooth for a tooth.” 39But I say to you, Do not resist an evildoer. But if anyone strikes you on the right cheek, turn the other also; 40and if anyone wants to sue you and take your coat, give your cloak as well; 41and if anyone forces you to go one mile, go also the second mile. 42Give to everyone who begs from you, and do not refuse anyone who wants to borrow from you." (Matthew 5:38-42)
The Catechism of the Catholic Church does however, describe self preservation (self defense?) as a legitimate cause for war. The Catechism seems to also require that the actions of self defense be proportional to the threat from the aggressor. (CCC. 2259-2267)
Now, in the case of Japan it seems clear that when the USA went to war in 1941 it was reacting to the Japanese attack on Pearl Harbor. In fact it seems clear that Japan was engaged in an aggressive war of conquest in the Pacific at that time. So I suspect that most people would say that initial American participation in the war was justified. However, was the use of atomic bombs justified in 1945. By 1945 Japan no longer had the means to wage aggressive war (although the Japanese army in China had been largely untouched by the war). The use of atomic weapons and the massive destruction of life which followed could hardly be described as a proportional response to the Pearl Harbor attack. So the justification that Hiroshima was simple retaliation for Pearl Harbor does not work from a moral point of view. There is however, the reality that even though Japan was clearly defeated by the summer of 1945 the country was still in the control of a fanatical band of militarists who would not accept defeat and who were prepared to sacrifice countless lives defending the Japanese homeland against invasion by using "kamikaze" tactics. So, the loss of lives at Hiroshima might be proportional to the loss of lives, both Japanese and American, that would be the consequence of an invasion of the Japanese home islands. So, if you accept that the defense of human lives (remember the continuing war in China) required the surrender of Japan and if you accept that the only way to achieve this was going to be by invasion then the use of the atomic bomb on Hiroshima might be justified. The attack on Nagasaki seems to be a different question to me.
Realpolitik is foreign policy based not on principles of morality but on calculations of power and of the national interest. Here is where the decision to attack Nagasaki (and to a certain extent Hiroshima) comes from. After Yalta at least some people saw the USSR as the future threat to the USA and to world peace. The USA had spent at lot of resources to acquire atomic weapons and they had no reason to believe that the Soviets were close to their own atomic bomb. Therefore it was in the US national interest to demonstrate (to everyone, not just the Japanese) the power of this weapon and the importance of it in determining the future of world politics.
The role of the USA in world affairs can sometimes seem quite ambiguous. There is no doubt that sometimes the Americans have been the defenders of freedom and truth (like in World War Two). There have also been times when their decisions have been based primarily on the notions of power and self-interest. I think that one of the things that explains the difference between the war in Iraq and in Afghanistan is this distinction. In Afghanistan the outside military involvement has been primarily to depose and rogue government that was supporting terrorism and to restore stability to a failed state. The motives in Iraq however, seem much more confused and this is what contributes to the controversy over that war.