Sixty years ago this month the United States dropped atomic bombs on the Japanese cities of Hiroshima (6 August 1945) and Nagasaki (9 August 1945). Those bombings may have helped end World War II -- thereby saving thousands of lives -- by forcing a quick Japanese surrender. But some historians now wonder, was it really necessary to drop so destructive a weapon on two crowded cities?

"On the cruisers and destroyers and battleships, our heavy batteries once more leveled against the Jap-studded hills of Okinawa.  The barking 20's and 40's sent streams of fiery lead into the world's last alien sky."

In 1945, with war raging in the Pacific, U.S. President Harry Truman was contemplating an invasion of Japan.

Robert James Maddox is a retired professor of history, and author of Weapons for Victory: The Hiroshima Decision.  He says Truman's decision to drop the atom bomb, rather than invade Japan, was never really in doubt.

"The predictions were [fighting] would be enormously bloody because the Japanese were amassing troops, they had millions, literally millions of troops, so estimates ran up to 500,000 American deaths or more in the event we had to undertake this," he said.  "There really wasn't so much a decision made to use the bomb as it was a question, as one official put it, of when the bomb would be ready."

"8:15 in the morning found a 400 pound [180 kilogram] bomb, with a destructive force of 20,000 tons of TNT mushrooming up over the stunned enemy city.  To the frightened inhabitants, the end of the world had come."

When the first Atomic bomb ever used in war exploded over the city of Hiroshima, between 70,000 to 90,000 people were instantly incinerated.

"Japan could not believe the tragedy that had befallen it.  But then three days later, a flock of B-29's soared over the seaport of Nagasaki."

A second bomb destroyed another 40,000 to 60,000 Japanese, mostly civilians, at Nagasaki. In the years that followed, tens of thousands more would die from the after effects of those blasts.

Japan surrendered days later. The war was over, and the U.S. had won. Avoiding a land-invasion of Japan was believed at the time to have saved thousands of lives. 

But author Kai Bird, who wrote a biography of Robert Oppenheimer -- one of the developers of the weapon -- says even Oppenheimer questioned the morality of the decision to use the bomb. "He said that this was a weapon of terror that had been used on a virtually defeated enemy, an already virtually defeated enemy," noted Mr. Bird.  "This is an extraordinary thing for the man who invented the weapon to say only months after the use of it."

Some critics today believe Japan was about ready to surrender, and that the Truman administration's real motivation may have been to project its newfound military might.  "And that it would also be useful to send a message in the post war period that America had this enormous weapon," added Mr. Bird, "and that, he referred to it as a weapon in our back pocket, that would be sending a message, a diplomatic message to the Soviets."

More traditional historians, like Robert Maddox, take issue with that analysis. "The so-called revisionist approach is that dropping the bomb was not the last chapter of World War II, it was the opening chapter of the Cold War," said Mr. Maddox, "and that when we dropped the bombs, we knew that Japan would surrender if only we gave them a chance, but that we dropped it anyhow in order to impress the Soviets.  That's nonsense."

It is still a contentious issue. Historians may still be arguing about the decision to drop the bomb 60 years from now.