1. Nuclear timeline:
August 1939: Albert Einstein writes FDR, discusses the possibility of creating a controlled nuclear reaction
July 16, 1945: the first nuclear bomb is tested in Los Alamos, New Mexico
Late July, 1945: at Potsdam, Germany, Pres. Truman issues an ultimatum: Japan must surrender or be destroyed by “new and terrible weapons”
August 6, 1945: the first atomic bomb is dropped on Hiroshima, a manufacturing city of 350,000; up to 80,000 die immediately
August 8, 1945: the Soviet Union declares war on Japan and invades Manchuria (Chinese land occupied by Japan)
August 9, 1945: the second bomb drops on Nagasaki

2. In 1994 the Smithsonian prepared to remember the Enola Gay and the atomic bombs:
As early as 1993 the Smithsonian Institution began to build an exhibit that would display the bomber and, they hoped, provoke discussion and reexamination over the use of the bombs.
Leaked previews of the exhibit’s content brought protests from war veterans and, consequently, Congress:

3. Enola Gay, continued:
“The atomic bombing of Hiroshima on August 6, 1945, and of Nagasaki three days later, were the first—and thus far—the only use of nuclear weapons in anger. Although mankind has lived with war and violence throughout its history, the atomic bombings announced the arrival of a new and qualitatively different peril, one that still threatens humanity: sudden, mass and indiscriminate destruction from a single weapon. From the vantage point of fifty years after these events, the atomic bombings and the end of World War II in the Pacific thus mark a turning point, an historic crossroads.”

4. Gar Alperovitz’s viewpoint:
This historian’s book Atomic Diplomacy was one of the first to argue that the bombs were not justifiable, and that Japanese surrender would have occurred anyway. As his title suggests, historians like Alperovitz also argue that a prime motivating factor for the bombs’ use was to manage the soon-to-be-ex ally, the Soviet Union. He continues to debate:
“This weekend marks the 60th anniversary of the August 6, 1945 bombing of Hiroshima. One might think that by now historians would agree on all the fundamental issues. The reality, however, is just the opposite: All the major issues involved in the decision are still very much a matter of dispute among experts. An obvious question is why this should be so after so many years.” (August 3, 2005)
Key article: “Hiroshima: Historians Reassess”

5. Robert James Maddox’s argument:
Of the historians who continue to argue that the bombings were justifiable, Dr. Maddox is among the most commonly cited. His works, he suggests,
“demolish revisionist positions on a number of issues: That Japan was ready to surrender, that expected casualties for the invasion of Japan were far lower than Truman would later claim, and that Truman wanted to keep the Soviets out of the war.”
Relevant article: “The Biggest Decision: Why We Had to Drop the Atomic Bombs,” from American Heritage
(1994)

6. Pro: The U.S. was justified in dropping the atomic bombs:
“Although a peace faction within the government wished to end the war … militants were prepared to fight on regardless of consequences. They claimed to welcome an invasion of the home islands, promising to inflict such hideous casualties that the United States would retreat from its announced policy of unconditional surrender. The militarists held effective power over the government and were capable of defying the emperor, as they had in the past, on the ground that his civilian advisers were misleading him.”

7. Con: The U.S. was not justified in using the atomic bombs:
a top-secret April 1946 War Department study, “Use of Atomic Bomb on Japan,” declassified during the 1970s but brought to broad public attention only in 1989, found that "the Japanese leaders had decided to surrender and were merely looking for sufficient pretext to convince the die-hard Army Group that Japan had lost the war and must capitulate to the Allies."

8. Pro:
“The Japanese could be expected to defend their sacred homeland with even greater fervor, and kamikazes, flying at short range, promised to be even more devastating than at Okinawa, The Japanese had more than 2,000,000 troops in the home islands, were training millions of irregulars, and for some time had been conserving aircraft that might have been used to protect Japanese cities against American bombers.”

9. Con:
… Political scientist Robert Pape's study, "Why Japan Surrendered," which appeared in the Fall 1993 issue of International Security, details Japan's military vulnerability, particularly its shortages of everything from ammunition and fuel to trained personnel: "Japan's military position was so poor that its leaders would likely have surrendered before invasion, and at roughly the same time in August 1945, even if the United States had not employed strategic bombing or the atomic bomb."

10. Pro:
In “In his memoirs Truman claimed that using atomic bombs prevented an invasion that would have cost 500,000 American lives. Other officials mentioned the same or even higher figures. Critics have assailed such statements as gross exaggerations designed to foreswear scrutiny of Truman's real motives. They have given wide publicity to a report prepared by the Joint War Plans Committee (JWPC) for the chiefs' meeting with Truman. The committee estimated that the invasion of Kyushu, followed by that of Honshu, as the chiefs proposed, would cost approximately 40,000 dead, 150,000 wounded, and 3,500 missing in action for a total of 193,500 casualties.”

11. Con:
“Over the last decade, scholars of very different political orientations … have all separately examined World War II U.S. military planning documents on this subject. These documents indicate that if an initial November 1945 landing on Kyushu had gone forward, estimates of the number of lives that would have been lost (and therefore possibly saved by use of the atomic bombs) were in the range of 20,000 to 26,000. In the unlikely event that a subsequent full-scale invasion had been mounted in 1946, the maximum estimate found in such documents was 46,000.”

12. Pro:
“A careful reading of the MAGIC intercepts … provides no evidence that retention of the emperor was the sole obstacle to peace. What they show instead is that the Japanese Foreign Office was trying to cut a deal through the Soviet Union that would have permitted Japan to retain its political system and its prewar empire contact. Even the most lenient American officials could not have countenanced such a settlement.”

13. Con:
“The other option that seemed likely to bring an end to the fighting concerned the Soviets. Joseph Stalin had promised to enter the war against Japan roughly three months after the May 8 defeat of Germany, which put the target date on or around August 8. Earlier in the war, the United States had sought Russia's help primarily to pin down Japanese armies in Manchuria and thus make a U.S. invasion of the home islands easier. By midsummer … U.S. military planners believed the mere shock of a Red Army attack might be sufficient to bring about surrender and thus make an invasion unnecessary.”