By John Lewis Gaddis, The Penguin Press, New York, N. Y., 784 pages, $39.95.

‘A dangerous man,” John Foster Dulles called George F. Kennan in 1950. Kennan, by then an experienced diplomat, had come out for the admission of Communist China to the United Nations. A new, authorized, prize-winning biography by Professor John Lewis Gaddis of Yale suggests that the real danger that Kennan posed was to the brand of Cold War diplomacy that Dulles preached and later came to practice.