Is not a happy one, of course, but Mr. Cleveland, a former Assistant Secretary of State and the current Ambassador to NATO, has done his best here to minimize the unpleasant aspects of the role. Subtitled ""American Power in a World of Disorder,"" his brief book is in fact an uncommonly persuasive defense of and plea for more support for U.S. foreign policy since World War II. The central objective of that policy, as he sees it, ""is to place our power in the service of a world of diversity."" Such a task is not easy, nor is explaining everything the U.S. has done or failed to do through the world in the past twenty years in such terms. It requires him to speak as if there really were a solid consensus in the nation, or the Administration, or even the State Department, as regards foreign affairs. After a while one wonders who, actually, besides Mr. Cleveland is included in his ubiquitous first-person plural. He says he represents those who like ""to play an imaginary game of Presidential solitaire with a stacked deck."" ""A diplomat,"" he quotes Harold Nicolson as saying, ""often has to choose between committing an indiscretion and uttering a cliche."" Long before the reader has reached the back cover, he knows which choice Mr. Cleveland has made.