From the beginning of John Kennedy's Administration into this fifth year of Lyndon Johnson's presidency, substantially the same small group of men have presided over the destiny of the United States. In that time they have carried the country from a limited involvement in Vietnam into a war that is brutal, probably unwinnable, and, to an increasing body of opinion, calamitous and immoral. How could it happen? Many in government or close to it will read the following article with the shock of recognition. Those less familiar with the processes of power can read it with assurance that the author had a firsthand opportunity to watch the slide down the slippery slope during five years (1961-1966) of service in the White House and Department of State. Mr. Thomson is an East Asia specialist and an assistant professor of history at Harvard.