THE AMBASSADOR HAD just had a long private meeting with President Harry S Truman, in office less than six weeks following the death of Franklin D. Roosevelt. Truman had told him two extraordinary things: First, if all went well, the United States would soon possess a weapon of awesome and hitherto unknown power.

Charging him with "utmost secrecy," Truman revealed something "which I have not told anybody" -- that he had decided to postpone negotiations with Stalin on the shape of the postwar world until he knew for sure whether the weapon really worked.

"I was startled, shocked and amazed," Joseph E. Davies, former U.S. envoy to the Soviet Union, wrote in his diary on May 21, 1945 after the meeting. In an asterisked footnote he added: "Uranium -- for reason of security I will have to fill this in later."

On July 16, the first atom bomb was tested successfully at Alamogordo, N.M. On July 17, Truman sat down to talk with Stalin. And on Aug. 6, a bomb would fall on Hiroshima, ultimately killing an estimated 130,000 Japanese and changing the world.

Now, 40 years later, revelations based on privately held and previously classified information continue to illuminate the complex decision-making that led to the destruction of Hiroshima and Nagasaki.

Most Americans assume the reason Hiroshima and Nagasaki were destroyed was simply to prevent a costly invasion of Japan.

However, the newest documents have strengthened the theory that other considerations -- especially the new weapon's impact on diplomacy toward the Soviet Union -- were involved.

The invasion of Japan -- which President Truman claimed might cost up to a million casualties -- was scheduled to begin on Nov. 1 with a landing on the island of Kyushu, with a full invasion in the spring of 1946. (Documents of the time suggest that many planners foresaw far fewer casualties.)

But by the mid-summer of 1945 Japan was in a very bad way. How allied intelligence understood the situation at the time was detailed in a report to the American and British Combined Chiefs of Staff, made public in 1976:

"The increasing effects of sea blockade and cumulative devastation wrought by strategic bombing . . . has already rendered millions homeless and has destroyed from 25 percent to 50 percent of the built-up area of Japan's most important cities . . . . A conditional surrender . . . might be offered by them at any time . . . ."

The Japanese code had been broken early in the war. Faint peace feelers appeared as early as September 1944.

In July, Secretary of the Navy James V. Forrestal's diary described the latest cables as "real evidence of a Japanese desire to get out of the war . . . ."

Forrestal was referring to a message from Togo to his ambassador in Moscow instructing him to see Molotov before he and Stalin left to meet Truman at the Potsdam Conference. The Japanese envoy was "to lay before him the emperor's strong desire to secure a termination of the war."

Forrestal noted that "Togo said further that the unconditional surrender terms of the Allies was (sic) about the only thing in the way . . . ."

Discussion of surrender was also underway through a channel in Switzerland. In a recently discovered memo dated May 12, William J. Donovan, director of the Office of Strategic Services, told Truman that an OSS source had "talked with Shunichi Kase, the Japanese minister to Switzerland . . . . Kase expressed a wish to help arrange for a cessation of hostilities . . . ."

Donovan reported the same judgment as that contained in the intercepted cables -- a slight change in the surrender formula seemed the only remaining issue: "One of the few provisions . . . would be the retention of the emperor . . . ."

Did top U.S. officials understand the import of the cables? There was, to be sure, the possibility that the initial feelers were without substance. However, Truman's diary, discovered in 1978, terms the key intercepted message "the telegram from Jap emperor asking for peace."

Adm. William D. Leahy, who served as chief of staff to the President and presided over the Joint Chiefs of Staff, wrote in his diary in mid-June that "at the present time . . . a surrender of Japan can be arranged with terms that can be accepted by Japan and that will make fully satisfactory provision for America's defense against future trans-Pacific aggression." Afterwards, Leahy would reflect that "the use of this barbarous weapon at Hiroshima and Nagasaki was of no material assistance in our war against Japan . . . ."

Likewise, Eisenhower would later state that "it wasn't necessary" to hit the Japanese "with that awful thing." On July 20, 1945, in front of Gen. Omar Bradley, he advised Truman of his objections.

There is some confusion as to precisely how other top military figures felt, particularly in the crucial last month before Hiroshima. There is no doubt, of course, that they approved planning for an invasion.

The important question is whether by July and early August military planners still believed an invasion would be required if the atomic bomb was not used.

Adm. Ernest J. King, commander in chief of the U.S. Fleet, had for much of the war argued that naval blockade would secure unconditional surrender without an invasion.

The top Army air forces commander, Gen. H.H. "Hap" Arnold, said unconditional surrender could be won by October. He outlined the devastation that would hit the Japanese population, with its enormous casualties.

"Japan, in fact, will become a nation without cities, with her transportation disrupted and will have tremendous difficulty in holding her people together for continued resistance."

Precisely how the leading Army figure, Gen. George C. Marshall, felt is not entirely clear. On the one hand, Marshall pressed forward on invasion planning, but he also urged changing the surrender formula and, as we shall see, advised of the importance of a Russian declaration of war.

As for the troops in the field: "Every individual moving to the Pacific," Marshall said, "should be indoctrinated with a firm determination to see it through."

Once the new weapon had been proven, the military leaders went along with the president's decision to use it. But this fact has often led subsequent observers to confuse approval with the question of whether, as Eisenhower put it, the weapon was still deemed "mandatory as a measure to save American lives." Strategy for the bomb was in any event largely handled outside the normal chain of command by the president and his advisers.

Did the president understand the possibility that the atomic bomb was not required to prevent an invasion? On this question there is much dispute. However, the documents now available make it very difficult to believe he did not.

First, Truman was repeatedly advised that a change in the unconditional surrender formula allowing Japan to keep the emperor seemed likely to end the war. There is also documentation -- from the diaries of Secretary of War Henry L. Stimson, acting Secretary of State Joseph C. Grew and from British Prime Minister Winston Churchill -- confirming that the president did not regard such a change as major. And in the end, of course, he did make such a change after the bomb was used.

It is sometimes argued that the Japanese military would have prevented a surrender had the atomic bomb not been used. But this argument usually assumes there would have been no change in the surrender formula. Given the right terms, as Leahy put it, "We were certain that the Mikado could stop the war with a royal word."

Of course, the president preferred not to alter the terms if possible.

The idea that the atomic bomb had to be used to avoid an invasion turns on whether or not there were other options.

As early as September 1944, Churchill felt the Japanese might collapse when Russia entered the war. On May 21, 1945, Secretary of War Stimson advised of the "profound military effect" of Soviet entry.

In mid-June, Marshall advised the president that "the impact of Russian entry on the already hopeless Japanese may well be the decisive action levering them into capitulation at that time or shortly thereafter if we land in Japan."

A month later the Combined British-U.S. Joint Chiefs of Staff discussed the Russian option at Potsdam. Gen. Sir Hastings Ismay summarized the Combined Intelligence Staffs' conclusion for Churchill: "If and when Russia came into the war against Japan the Japanese would probably wish to get out on almost any terms short of the dethronement of the emperor."

Did the president also understand the advice that the Russian declaration of war was likely to bring about capitulation?

After his first meeting with Stalin on July 17, 1945 -- three weeks before Hiroshima -- the president noted in his diary:

"He'll be in the Jap war on August 15th. Fini Japs when that comes about."

It is clear that the president preferred to end the war without Russian help, but that does not mean that he had no alternative but to use the atomic bomb. We now know he rejected Russian help for political, not military, reasons.

The original planning date for Russian entry into the war was August 8. Hiroshima was bombed on August 6 and Nagasaki on August 9.

The person for whom the linkage between the atomic bomb and strategy towards Russia was most direct was Secretary of State James F. Byrnes -- Truman's chief adviser both on diplomacy and on the atomic bomb.

Byrnes was a complex, secretive, even devious politician. In his diary Truman refers to him at this time as "conniving."

There is unmistakable evidence that Byrnes tried to rewrite the historical record, in part by destroying documents, in part by literally rewriting the private diaries of his assistant, Warren Brown -- and passing them off to official government archivists as authentic.

In any case, Forrestal's diaries, show Byrnes "most anxious to get the Japanese affair over with before the Russians got in . . . ." It was also Byrnes who formally proposed that the bomb be targeted on a factory surrounded as closely as possible by workers' housing to achieve maximum psychological effect.

Ambassador Davies, who was "shocked, startled and amazed" when told of the decision to postpone talks with Stalin, was disturbed by "Byrnes' attitude that the atomic bomb assured ultimate success in negotiations . . . ." On July 28, 1945 Davies warned him that "the threat wouldn't work, and might do irreparable harm."

Byrnes was particularly worried that if the Russians entered the Japanese war they would get control of Manchuria and north China. He was also concerned about Eastern Europe. Roosevelt had selected Byrnes -- his "assistant president" at the time -- as the leading public advocate and defender of the famous Yalta agreement which promised democracy and free elections in Eastern Europe.

Though at Yalta Byrnes participated in cutting the teeth out of language that would have made the agreement more than a statement of general intentions, recent research indicates he hoped the atomic bomb would enforce in practice what had been signed away in principle.

According to atomic scientist Leo Szilard, who met with Byrnes on May 28, 1945 -- 10 weeks before Hiroshima: "Mr. Byrnes did not argue that it was necessary to use the bomb against the cities of Japan in order to win the war . . . " Byrnes "was concerned about Russia's postwar behavior.

"Russian troops had moved into Hungary and Rumania; Byrnes thought it would be very difficult to persuade Russia to withdraw . . . and that Russia might be more manageable if impressed by American military might.

"I shared Byrnes's concern . . . " Szilard observed, "but I was completely flabbergasted by the assumption that rattling the bomb might make Russia more manageable . . . ."

There is no evidence Byrnes used the atomic bomb as an explicit threat, but a month after the Potsdam meeting with Stalin, for example, Stimson talked with him at the White House, and noted in his diary: "I found that Byrnes was very much against any attempt to cooperate with Russia. His mind is full of his problems with the coming meeting of foreign ministers, and he looks to having the presence of the bomb in his pocket, so to speak, as a great weapon . . . ."

Byrnes, who previously had been senator from South Carolina, was on very intimate terms with the president. He had, in fact, acted as Truman's mentor when he went to the Senate from Missouri. Roosevelt had also seemingly selected Byrnes to be vice president in 1944, switching only at the last minute to Truman.

One of the reasons Truman made Byrnes secretary of state was that this move put Byrnes next in line of succession for the presidency after Truman moved up from vice president.

On May 3, 1945, Truman also asked Byrnes to be his representative on the "Interim Committee" studying atomic strategy -- and there were numerous meetings between the two men throughout the summer.

Truman and Byrnes left Washington together on July 7 to meet with Stalin at Potsdam, where Stimson complained that Byrnes was "hugging matters pretty close to his bosom."

Before the Potsdam conference Truman was also advised by Stimson: "We shall probably hold more cards in our hands later than now." During the conference Truman was enormously bolstered by the successful atomic test. "Now I know what happened to Truman yesterday," Churchill observed. "I couldn't understand it. When he got to the meeting after having read this report (of the atomic test) he was a changed man."

"He told the Russians just where they got on and off and generally bossed the whole meeting."

He also told Stalin that America had developed a powerful new weapon, but did not specify that it was atomic.

There are still many unanswered questions about the decisions made during the month before Hiroshima. However, there is little doubt about some things. Had the United States so desired, either the forthcoming Russian declaration of war or a change in the surrender formula (or both together) seemed likely to end the war without the atomic bomb. There was also plenty of time to use the weapon if these options failed in the three months before the Kyushu landing.

"The historic fact remains, and must be judged in the aftertime," Churchill subsequently observed, "that the decision whether or not to use the atomic bomb . . . was never even an issue." It is possible that top policy makers, especially the president, simply wanted to leave no stone unturned to end the war.

However, in view of what we now know about Japan's attempt to surrender, military factors alone appear inadequate to explain the choice.

As historian Martin Sherwin put it, the idea the atomic bomb would help make Russia manageable both in Asia and in Europe was an important consideration -- "inextricably involved."

In mid-May America's leaders had postponed negotiations with Stalin, basing their strategy on the assumption the bomb would strengthen their hand. Thereafter, some of those most intimately involved in diplomacy -- unlike some of the top military figures -- apparently were either unable or unwilling to understand the significance of the June and July information on Japan's collapse.

The evidence that diplomatic considerations were very important is especially clear in connection with the president's closest adviser, Byrnes. Nevertheless, 40 years after the fact some government documents still remain classified. It may be that when these are finally released -- perhaps when still other diaries are discovered -- we will know the full story.