HARRY TRUMAN certainly knew how to downsize: He simply picked up a pen and wrote an end to the giant central intelligence organization that had burgeoned during World War II. No musing about reordering its priorities or finding new roles and missions. He named no task force to redefine the massive Office of Strategic Services, the nation's first centralized intelligence organization. On Sept. 20, 1945, 18 days after Japan's formal surrender ended World War II, Truman signed the OSS out of existence effective Oct. 1. The OSS director said he expected to complete cutbacks within six months; instead, Truman gave him 10 days to close up shop. Yet less than four months later, the president set up a Central Intelligence Group to provide him with daily intelligence summaries. And less than two years after succeeding to the presidency, he signed a proposal to Congress to create what in September 1947 would become the CIA - and would bear a striking resemblance to the OSS he sent packing. Did Truman have an epiphany? What explains his rapid turnaround? The anticipated declassification of more intelligence documents may help scholars clarify what Truman really wanted an intelligence organization to be. Until then, the best guess is that his actions reflected a combination of wishful thinking and personal pique, followed by a pragmatic response to the dramatic march of international events. Evidently Truman hoped - and wanted to believe - that peace would reign in the wake of World War II. Perhaps there was a touch of naivete in his notion that an OSS was no longer needed. His temper doubtless was a factor as well. He instantly disliked the head of the OSS, Gen. ''Wild Bill'' Donovan, an Eastern establishment patrician who stressed his closeness to Roosevelt in urging Truman to retain the OSS. The new president's perception of a swashbuckling OSS reflected his view of Donovan. Truman relished the opportunity to put Donovan out of business along with the organization he touted. But Truman clearly had in mind the need for a centralized organization to coordinate intelligence for the president and had solicited proposals for creating such a capability even before he did away with the OSS. In his executive order eliminating the OSS, Truman noted that the United States needed ''a comprehensive and coordinated foreign intelligence program.'' But he initially left the task to interdepartmental cooperation - never solid ground on which to build. In December 1945, an impatient Truman was asking to see plans. Seizing the draft that looked simplest and most workable, after the holidays he created the Central Intelligence Group. He wanted to ensure that he received intelligence on a timely basis, and by February 1946 was reading a daily summary that was the genesis of today's daily brief. If this rudimentary organization was all Truman wanted, he came to realize there was more that he needed. Within six months, the Central Intelligence Group got an independent budget and its own work force, rather than one seconded from other departments. It was authorized to collect, analyze, and collate intelligence. By its first birthday - before the Truman Doctrine launched the policy of containing communist expansion - the CIG had grown from about 100 employees to over 1,800. As part of a broader plan for a more coherent national security establishment, the CIA succeeded the Central Intelligence Group in September 1947. In December, the first National Security Council directive authorizing covert operations assigned the CIA the mission of helping to prevent a communist takeover in Italy. The following June, Truman signed another NSC directive spelling out the fact that ''overt foreign activities of the US government must be supplemented by covert operations'' and giving CIA responsibility for them. Covert action and the CIA grew like topsy. But the CIA was not the only government agency experiencing cold-war growth. When the Soviets successfully launched their Sputnik satellite, the National Aeronautics and Space Administration had 900 employees. Four years later it had 33,000. Although not on the same order of magnitude, the Korean War was the CIA's Sputnik. Korea brought demands that sent soaring the number of CIA personnel engaged in covert operations. What the CIA had become by the 1960s was almost certainly not what Truman intended or what anyone foresaw. In 1963, years after his presidency ended, Truman lamented in print that the ''quiet intelligence arm of the president has been so removed from its intended role that it is being interpreted as a symbol of sinister and mysterious foreign intrigue.'' Senior CIA officers were aghast. An indignant Allen Dulles - the former CIA director and champion of covert action, who had retired in the aftermath of the Bay of Pigs disaster - traveled to Independence, Mo., to remind Truman that he had personally approved the covert-action programs during the war in Korea. Dulles' assertion and Truman's reflection may not be a comfortable fit but can stand uncorrected. Truman did approve covert actions, but while he conceded the need, in all probability he did not favor an emphasis on them. Perhaps his decisiveness about OSS was precipitous. Much of America's intelligence capability had to be rebuilt as a result. It is impossible to say how much expense might have been spared if the OSS had not been so summarily dismantled, or how much was improved by rebuilding its successor through trial and error. Truman penned his last words on the subject a year after criticizing the CIA's covert-action image. On a photograph that hangs in the agency's main hallway today, he wrote: ''To the Central Intelligence Agency, a necessity to the President of the United States, from one who knows.''