AT 8:15 A.M. AUG. 6, 1945, TOKYO TIME, the bomb bay doors of the Enola Gay swung open and the bomber's 9,000-pound cargo began its fall on the Japanese city of Hiroshima, which was clearly visible 31,600 feet below.

The pilot of Enola Gay, Col. Paul W. Tibbets Jr., banked the B29 sharply to the right. About 45 seconds later most of Hiroshima disappeared in a blinding flash of heat and light. The tail gunner warned that he could see the shock wave from the awesome explosion approaching, and it hit the big bomber with a bang that shook it like a hurricane.

"There was the mushroom cloud growing up and we watched it blossom," Tibbets recalled. "It was black and boiling underneath with a steam haze on top of it. And, of course, we had seen the city when we went in, and there was nothing to see when we cme back. It was covered by this boiling, black-looking mess."

Nearly 100,000 people in Hiroshima were killed instantly. Thousands more subsequently died agonizing deaths from burns and radiation poisoning from the first atomic bomb, a six-foot-long uranium bomb nicknamed "Little Boy" that packed JAMES R. DICKENSON is a political writer on the National staff of The Washington Post. 14,000 tons of TNT.

That devastating blast, whose power surprised even the bomb's creators, set in motion the final events that led to the Japanese surrender nine days later and ended World War II, the most destructive war in history.

In a ghastly paradox, at the same instant that the blast assured the end of the most diligent and efficient slaughter humans had ever inflicted on each other, it marked an exponential leap in man's capacity to destroy himself. Man had assumed the power of the gods and his punishment would be that forevermore he would live on the edge of the abyss, never to be free of the certainty that he possessed the capability many times over of incinerating the planet and exterminating himself.

The awesomeness of that step prompted J. Robert Oppenheimer, leader of the scientific team that developed the bomb, to quote from the Bhagavad-Gita after the secret test blast at Alamogordo, N.M., July 16: "I am become Death, Shatterer of Worlds."

THREE DAYS after Hiroshima, on Aug. 9, the second nuclear weapon, a 10,000-pound plutonium bomb nicknamed "Fat Man," was dropped on Naga000 more Japanese. The next day a tearful Emperor Hirohito informed the equally emotional cabinet of his wish that the Allies' surrender terms be accepted and his people be spared any further suffering. On Aug. 15, Japan surrendered, despite the efforts of a group of fanatical army officers who attempted a coup in the hope of destroying Hirohito's taped radio address to the people and persuading him to change his mind.

No formal peace treaty charted the future, but the settlement has remained remarkably stable as the nuclear superpowers, the United States and the Soviet Union, have held the world in an uneasy nuclear balance for 40 years despite their mutual antagonisms.

The outlines of the settlement in Europe, dictated primarily by the Soviet Union's demand that it dominate Eastern Europe and that there be a control on Germany, hasn't changed in 40 years. With the exception of the U.S. wars in Korea and Vietnam, Asia has been remarkably stable and economically dynamic over the same period. The contrast with the aftermath of the First World War is striking. As Prof. Stanley Hoffman of Harvard University notes, "The Treaty of Versailles (the World War I settlement) collapsed within 15 years and with disastrous consequences."

WORLD WAR II is unique among wars because of the overwhelming agreement among historians on its necessity and on which side was right, which wrong. There is no debating the unequivocal threat to humanity and civilization that was posed by the Nazis, abetted by the Japanese. The lethal combination of nationalism, the aggressive, totalitarian governments of the Axis powers and modern technology turned the globe into a gigantic slaughterhouse.

Estimates of the total number of dead, both military and civilian, range from 40 million to 50 million. This includes the 6 million Jews who were murdered in the Nazi Holocaust plus millions of other nonbelligerents.

More than 30 million dead, half of them civilians, were from the eight major belligerent nations -- the Soviet Union, Germany,

Japan, China, Great Britain, France, the

United States and Italy, to list them by

magnitude of loss.

Millions more were wounded, impoverished, suffered the loss of loved ones

and had their lives tragically disrupted.

The loss in money is so astronomical as

to be practically meaningless.

The Soviet Union bore the brunt of

the war against Germany, with an estimated 7.5 million servicemen killed in action, 14 million wounded, and 10 million

to 15 million civilians dead. Thus, one in

every eight or nine of the Soviet Union's 1940 population of 170.5 million died in the Nazis' savage racist onslaught on the Eastern Front, which the Soviets commemorate as "The Great Patriotic War."

Memory of the savagery leads the Soviets to fear Germany even more than the United States and its nuclear arsenal. This fear lay behind the Soviets' determination to keep Eastern European buffer states between the U.S.S.R. and Germany. Some historians contend that the Soviets initially resisted the division of Germany because they had no control over West Germany, the stonger half, and feared it would be an unstabilizing force in Europe.

The United States lost 292,100 soldiers, sailors and marines killed in action, about one in every 450 of its 1940 population of 131.7 million, plus 571,822 wounded. Civilian casualties were negligible.

The British Commonwealth lost about 544,600 killed in action, about 398,000 of whom were from the United Kingdom, plus about 65,000 civilians killed. This is about one in 100 killed out of the UK's 1940 population of 46 million.

About 210,600 French were killed in action plus 108,000 civilian dead, about one in every 140 of the population of 42 million, plus 400,000 wounded. China suffered about 500,000 killed in action plus 1 million civilian dead, about one in 350 of its population of 458 million, with about 1.7 million wounded.

Germany paid a fearsome price for its war crimes, suffering casualties second only to the Soviets'. About 2,850,000 German military men were killed or missing in action and about 500, civilians died, one in every 24 of its 1940 population of 80 million. Another 7,250,000 were wounded.

Japan lost about 1.5 million killed in action plus 300,000 civilians dead, one in every 40, plus about 500,000 wounded in action, a killed-to-wounded ratio that reflects the Japanese's fanatic fight to the death in battle. Italy lost about 77,500 killed in action and 40,000 to 100,000 civilians, about one in 300 of its population, plus 120,000 wounded.

The ferocity of the killing is as dismaying as the boxcar numbers. The Nazis' genocidal murder of European Jewry is a horror almost too monstrous to assimilate. By the time of Hiroshima and Nagasaki, all the participants, Allies and Axis alike, had been hardened by the wholesale slaughter of the previous six years.

"By then, we'd gotten to the point where our sensitivities were dulled to wiping out cities," says Prof. David Kaiser, a historian at Carnegie-Mellon Institute of Technology.

Harry S Truman's decision to use nuclear weapons remains a matter of dispute. Historians disagree over the wisdom of insisting on unconditional surrender and on the Japanese willingness to surrender prior to Hiroshima, but 40 years later there is a consensus that the major factor in Truman's decision was to end the war as quickly as possible.

By the summer of 1945 the United States had plans for an 11-division assault on Kyushu at the beginning of November 1945, a force of 767,000 men going against Japanese forces of almost the same strength. U.S. leaders assumed that the casualty rate would be about the same as on Okinawa, about 35 percent, which would mean about 268,000 Americans killed or wounded.

The anticipated casualties for the assault on the main island of Honshu, scheduled for March 1, 1946, was 500,000 to 1 million dead plus the usual ratio of two or three wounded for each one killed.

U.S. leaders were dismayed by the high costs of the final Pacific island battles, which seemed to become increasingly ferocious. They knew there were thousands of kamikazes, suicide pilots, ready to defend the home islands when the invasion came. In addition, both soldiers and civilians were being trained to make suicidal "close quarter attacks" on tanks with Molotov cocktails and "shaped charges" that burned through armor. Some, including children, were to throw themselves under tanks with explosive charges strapped to their backs.

For the hundreds of thousands of young Americans staging for the assault in the Pacific from Hawaii to Okinawa or in transit after hard fighting in Europe, the news of Hiroshima and Nagasaki was a deliverance.

"I was a 21-year-old second lieutenant leading a rifle platoon," Paul Fussell wrote in The Washington Post on the 35th anniversary of the war's end. "When the bombs dropped and news began to circulate that 'Operation Olympic' would not, after all, take place, that we would not be obliged to run up the beaches near Tokyo assault-firing while being mortared and shelled, for all the fake manliness of our facades, we cried with relief and joy. We were going to live. We were going to grow up to adulthood after all."

Some historians argue that Japan would have surrendered without resort to the atomic bomb or invasion of the home islands. One of the most adamant is Gar Alperovitz, author of Atomic Diplomacy: Hiroshima and Potsdam.

Alperovitz argues that there was general agreement that the Japanese were defeated and that the top U.S. military leadership opposed use of the bomb. He also argues that Truman and the rest of the U.S. leadership were convinced that Japan would surrender when the Soviet Union declared war on her and if the Allies would agree to retention of Emperor Hirohito as Japan's spiritual leader.

He contends that Truman dropped the bomb primarily because Secretary of State James F. Byrnes argued that it would be a bargaining chip that could help "keep the Russians straight" and force them to make concessions in Eastern Europe.

Many historians, however, doubt that the Japanese would have surrendered without use of the bomb, given the samurai tradition of devotion to duty, self-sacrifice for the emperor and the fanaticism of the Japanese military in every battle.

Diplomatic traffic between Tokyo and the Japanese ambassador in Moscow, which U.S. codebreakers were intercepting, indicated that the Japanese were balking on the insistence on unconditional surrender and would fight on as long as that was the only alternative. They also hoped that the Soviets might help mediate more acceptable terms.

A major reason for the Allies' insistence on unconditional surrender was the belief that the World War I victors' decision to allow the defeated German government to remain standing (except for the abdication of the kaiser) was a factor in Germany's powerful and malevolent rebound. Many of the Allies feared the tragedy might be repeated if surrender terms were too easy.

"Truman inherited the momentum of the development of the bomb from (Franklin D.) Roosevelt and you can only assume that Roosevelt developed it because he intended to use it," says Prof. William E. Leuchtenburg of the University of North Carolina. "Still, it did break a barrier. And there's the fact that it was dropped on a nonwhite nation. Would we have used it on Germany?"

There is also disagreement over the need for the second bomb. No one knew beforehand, however, the destructiveness of the nuclear bombs or their psychological impact; Oppenheimer speculated that as many as 12 or 15 might be necessary.

On July 26, the Allies issued the Potsdam Declaration demanding that Japan surrender unconditionally or face complete destruction. Two days later, Prime Minister Suzuki rejected it.

The day after Nagasaki, however, the Japanese government announced its acceptance of the Potsdam Declaration with the proviso that "it does not compromise any demand which prejudices the prerogatives of His Majesty as a Sovereign Ruler."

U.S. officials drafted a compromise reply stipulating that from the moment of surrender "the authority of the Emperor and the Japanese Government to rule the state shall be subject to the Supreme Commander of the Allied powers" and that the "ultimate form of the Government of Japan shall be established by the freely expressed will of the Japanese people."

The surrender was decided at the final meeting of the Japanese Supreme War Council when Hirohito, who previously had played a passive role in government, decreed surrender because "I cannot stand putting my people to further suffering."

The council accepted the emperor's decision. He taped two radio broadcasts to the people, which were transmitted on Aug. 15, the date of surrender. He dispatched members of his fami to relay the surrender order to the garrisons in China, Korea and Manchuria.

On Aug. 14, however, a group of fanatical army officers seized control of the Imperial Guards Division, assassinated its commander and tried to find the radio tapes in the hope of changing Hirohito's mind. Loyal officers and troops overpowered them, and the emperor's call to surrender was broadcast although there were scattered uprisings. The Japanese people followed the lead of their emperor. When Gen. Douglas MacArthur and the first American occupation troops landed on Aug. 28, they were greeted by a docile populace, and on Sept. 2 the Instrument of Surrender was duly signed.

Few foresaw it at the time, but after a ferocious and in many ways racist war, the United States was about to acquire a strong democratic ally -- with Germany, two such allies.