IWAS somewhere in Switzerland, watching the World War II anniversary celebration on CNN, when it occurred to me that from my own experience as a reporter I could provide a firsthand account of why Harry Truman made the decision to drop the nuclear bomb that brought that war to an end. I was interviewing Truman in Kansas City, Mo., shortly after he had returned to private life, and at some point I asked him about that fateful decision. Truman, as is well known, could be prickly, particularly in dealing with a journalist who was pushing him in a direction where he didn't want to go. But a Truman who had been most cordial and chatty up to this point didn't change his easy, conversational tone one iota as he addressed this question. It wasn't that difficult a decision, he told me. He said he had turned to the military advisers to counsel him on what to do and that when they told him the bomb was a military weapon that would save thousands of American lives, the decision was obvious. Without further thought, he said, he gave his go-ahead to the use of the bomb in Japan and then, he added, ''I went to bed and got a good eight-hour sleep.'' Truman, it seemed, had looked at the decision in simple terms, much as he might have viewed some small military action he might have had to decide when he was a company commander in World War I. Indeed, it was much the way Truman always decided matters. He'd conclude what was the right thing to do and then do it. He was not a man, or a president, to stew over how to proceed. When Truman made that bombing decision he must have known that the powerful blast would kill thousands of Japanese civilians. But at that time he would not have known that a horrible radiation aftermath would take place. I have found in my files a yellowed, memo-sized letter with this printed at the top: ''From the Desk of Harry S. Truman.'' On it Truman had written: ''You've written a good article. I appreciate it.'' Signed: ''Harry Truman'' followed by (again in his handwriting) ''To Godfrey Sperling, Jr., Christian Science Monitor, July 17, 1953.'' There was no way that Truman would have foreseen how controversial that bombing decision would become in later years. Nor could he have anticipated the revisionist historians who now say he shouldn't have authorized the bombings and, furthermore, assert that the war would have ended quickly without the bomb. So he was in no way dealing with a heated controversy when he talked to me about his motives. There is a theory, put out now by some latter-day experts on World War II, that Truman used the bomb to send a message to the Russian leaders - to give the United States a better bargaining position in the peace negotiations that would come. I don't believe it. And I don't believe it simply because I looked into the face of Harry Truman and am confident that I was listening to a guileless fellow who was telling me what happened just the way it was. Frankly, I always thought Truman gave you the straight goods - as they might have said back in Missouri when he was in the haberdashery business. My story was treated by my newspaper as a new insight into Truman's thinking about the bomb and was, I think, picked up by the wire services. Truman later gave this bombing explanation to others in the press. After an hour or so, my get-together with Truman was over and I wrote at the time: ''At this point Mr. Truman stood, the interview over. ''Despite the folksy manner, his dapper dress, and his quiet voice, there is something in the way he rises, shoulders thrown back, chin up, that tells you softly but clearly you are in the presence of a former president.''