Was the US justified in using the atomic bomb against Japan in World War II?

The US was justified in using the atomic bombs against Japan.  Using the bombs saved tremendous numbers of American lives.  In addition, while the US may not have actually considered this at the time, the atomic bombs surely saved many more Japanese lives than they took. 

When a country is at war, its first responsibility is to win the war while taking the fewest possible casualties.  Of course, there are some rules about how the enemy must be treated, but the atomic bombs did not break these rules.  While we treat the atomic bomb as if it were somehow much different and much worse than conventional weapons, it is not clear that it really was.  For example, the Americans firebombed Tokyo on one night in 1945 and killed more Japanese than were killed immediately by either atomic bomb (but not more than both combined).  It is hard to see how the atomic bomb was much worse than this sort of bombing. 

In addition, the atomic bombs were surely better than what would have happened if the US had invaded the home islands.  It is estimated that as many as 100,000 or more civilians died in the American invasion.  If that many Japanese could be killed in the invasion of a small island, how many more would have been killed in an invasion of all of the main islands of Japan?

When we look at it like this, the decision to drop the bombs is justifiable.  What the bombs did was horrible.  What would have happened to American military personnel, Japanese military personnel, and Japanese civilians in an invasion of Japan would have been worse.

How did the US justify using atomic bombs on Hiroshima and Nagasaki?

It is, perhaps, historically inaccurate to say that the US justified its decision at the time that it was made.  At the time, there was almost no controversy over the use of the bomb.  Winston Churchill reported, for example, that it was simply assumed that the US would use any weapon that it had to win the war.  The idea that the US needed to justify the decision came mainly after the war.

The justification that has typically been given is that the atomic bombs saved lives.  If the United States had had to invade the home islands of Japan, the toll in human life would have been horrific.  There are estimates that a million Americans would have been killed or wounded. The number of Japanese casualties would have been much higher.  Using the bombs to end the war quickly, then, was undoubtedly horrible for the people of the two cities involved.  However, it likely saved the lives of many more people than it killed.  This is the major justification for the use of the bomb.

Was President Truman's decision to use atomic bombs against Japan justified?

The bombing of Hiroshima and Nagasaki gave the Japanese an excuse for surrendering unconditionally. Otherwise, they seemed prepared to fight for every inch of their homeland. Furthermore, once the United States had the atomic bomb it had to show that it was willing to use it. And after dropping the bomb on Hiroshima, it was necessary to drop another one in order to prove that the U.S. had a supply of such bombs. The U.S. wanted to prove this not only to Japan but to the world at large, and especially to the U.S.S.R. It is quite true that the atom bombs actually saved lives on both sides. The Americans could see what resistance the Japanese were capable of by observing what happened at Iwo Jima, Okinawa, and the suicide attacks on the fleet. Most Americans did not have the slightest idea that such things as atom bombs existed or were possible to make. It was all very much top secret. 

To make this argument, we should focus on the number of lives that were saved by this decision.  It is of course true that the people of Hiroshima and Nagasaki suffered terribly because of the bombs.  But it is likely that many more people would have suffered and died had the bomb not been used.  The military estimated that 1 million Americans would be killed or wounded in an invasion of Japan.  It is certain that Japanese casualties would have been even higher.  Therefore, the use of the bombs can be justified as a way of saving lives, both American and Japanese.

Was the dropping of the atomic bomb on Hiroshima justified?

The case for dropping the atomic bomb revolves around the idea that a commander does everything in his power (that is legal) to do away with unnecessary risks to the troops under his command.  Since dropping the bomb was clearly not illegal (any more than huge carpet bombing raids were illegal), it was right for President Truman to use the bomb to save lives.

In August of 1945, it seemed clear that an invasion of Japan would be horribly costly in terms of American lives (to say nothing of Japanese lives).  The fighting on Saipan, Iwo Jima, Okinawa, and other places showed that the Japanese would fight practically to the last man.  It was reasonable to expect that they would resist even more fiercely on their own home islands.  Therefore, an invasion (the most likely alternative to using the bomb) would have killed and wounded hundreds of thousands of Americans.  In addition, it would surely have killed more Japanese than the atomic bomb did.

For these reasons, dropping the bomb was justified.