Gorbachev in Midstream : WHY GORBACHEV HAPPENED: His Triumphs and His Failure, <i> By Robert G. Kaiser (Simon & Schuster: $24.95; 476 pp.)</i>

Writing a book about Mikhail Gorbachev these days is like trying to do a military history of a battle that has not yet ended. The ultimate outcome will color the analysis of all that has gone before. Victory will make the opening moves look ingenious and courageous; defeat will render them rash or poorly planned. History is a merciless judge.

Furthermore, Gorbachev’s tenure as Soviet leader has been a phenomenon of immense complexity, not at all susceptible to the easy caricatures that have been purveyed by some American politicians, commentators and columnists who have seen him as either just like his autocratic predecessors or just like us.

In truth, of course, Gorbachev is no Massachusetts Democrat; he is a veteran Communist Party official with political reflexes very different from ours. Nor is he a dictatorial charlatan; he has opened his society internally and removed its stifling weight from world affairs, most dramatically from Eastern Europe.

As a politician, he has displayed impressive skill in a closed system, and ineptitude as his country has fragmented into more pluralistic politics. He has tolerated criticism and tried to suppress it; he has dispersed power and tried to retain it.

To assess Gorbachev, then, is to accept a formidable task, which is reason enough to admire Robert G. Kaiser’s undertaking in “Why Gorbachev Happened.” Between the galley proofs and the hardcover, Kaiser changed his subtitle from “The Man and His Revolution” to “His Triumphs and His Failure,” an illustration of how hard it is to hit a moving target. A few months ago, Gorbachev’s legacy was revolution; today it is failure. It may actually turn out to be both, as Kaiser illustrates in a book that is much subtler and far less predictive than the finality of the word failure suggests.

Indeed, the strength of what Kaiser has done lies partly in his refusal to be categorical or dogmatic about the Soviet leader. The author does declare in his closing chapter, written just after last January’s attack by troops on Lithuanian demonstrators in Vilnius, that with the use of force, “the hopeful, high-minded Gorbachev era ended.” And he seems to pronounce Gorbachev dead as a liberal reformer, criticizing him as unable to grasp either economic issues or the powerful ethnic and national identities that tear at the fabric of Soviet life.

“The army, the KGB, the police, the still hidden but still powerful military-industrial complex, and remnants of the Party apparatus pushed Gorbachev off course at the very moment when he seemed to be triumphant,” Kaiser writes. “The wild, bucking horse he had been riding for seventy months finally threw him.”

But throughout his illuminating book, the author wrestles with the contradictions of the man, searching for the roots of Gorbachev’s competing impulses of reform and conservatism, those clashing values that coexist, side by side, both in the Soviet leader and in Soviet society as a whole.

Gorbachev the revolutionary and Gorbachev the apparatchik passed the baton back and forth to one another, the author says. “He switched between the two roles as he saw fit. . . . In the first five years these interludes of retrenchment probably smoothed the way for further change. Gorbachev could not have bulled ahead stubbornly without any pause or hesitation.”

Kaiser perceptively places the Soviet leader in the context of his society and its history. He sifts the record for telling details of personal biography, noting that both of Gorbachev’s grandfathers were arrested under Stalin, and that Gorbachev once described “his boyhood home as a ‘plague house’ after Grandfather Gopkolo returned . . . a house ‘where even relatives and close friends could not visit’ for fear of being associated with this ‘enemy of the people.’ ”

Later, in 1956, Gorbachev was in his first year after law school when Nikita Khrushchev shattered the icon of Stalin in his “secret speech” denouncing the dictator’s crimes. “There is no record of Gorbachev’s reaction to the speech,” Kaiser writes, “but his contemporaries who shared that experience are confident of its importance in his life, as in theirs.” In a wise observation, the author concludes, “As long as he was dismantling the Stalinist system, Gorbachev was working with the forces of history, not against them.”

Kaiser, now deputy managing editor of the Washington Post, approaches Gorbachev with the clear eye of a good reporter and the perspective of 20 years of Soviet-watching, beginning with his assignment as the Post’s Moscow correspondent from 1971 to 1974. His book from those years, “Russia: The People and the Power” was a warmly written journey through the human dimensions of Soviet society.

By contrast, “How Gorbachev Happened” is a lucid, chronological account of visible, high-level political events, sometimes made unavoidably sterile by Kaiser’s remoteness. Being in Washington, and having returned only occasionally to the Soviet Union (once to interview Gorbachev), Kaiser relies mainly on transcripts of speeches and legislative and party meetings; the dispatches of American correspondents, most notably David Remnick and Michael Dobbs of the Post; Soviet press reports, and some interviews with Soviet officials, who have given some new, inside details.

One virtue of the approach is to pay close attention to what Gorbachev has said over the years, a fruitful source of insight often overlooked. Although more specific footnotes would be helpful, the book contains an excellent chronology compiled by Jennifer Long.

In depending mostly on secondary sources, however, “Why Gorbachev Happened” misses some of the vigorous dynamics of behind-the-scenes negotiation, and it pays little attention to the political and governmental processes that Gorbachev has set in motion. The most glaring example is the account of the session of the Congress of People’s Deputies in March, 1990, which established the presidency and elected Gorbachev. The larger importance of the event is inadequately discussed: The Congress created not just a presidency but, for the first time, a limit to the term of office, a mechanism of succession and a strong legislature--in short, a structure of checks and balances modeled on American and French principles. Gorbachev has been only partly to blame for the failure of this new constitutional system to function well so far; in accordance with long tradition and in the midst of crisis, the legislature has not used its powers sufficiently, a key issue that would have been worth examining.

By putting Gorbachev at center stage, Kaiser has necessarily chosen to leave in the wings the ordinary Soviet citizen, whose reactions to the revolution have been integral to its fate. But even a book on Kremlin politics could deal more thoroughly with attitudes in the larger society, where there is ethical resistance to the profit-making that comes with private enterprise, where “democracy” often is seen as disorderly, where debate and compromise have not yet been developed, where no unifying belief commands allegiance any longer, where free speech has been discredited in many minds for failing to put food on shelves.

A few minor errors and omissions also crop up here and there. Grigory Romanov, the former Politburo member, is given the first name Georgi. George P. Shultz’s name is misspelled as Schultz . Nikolai Ryzhkov never gets replaced as prime minister by Valentin Pavlov, although the change was made in January.

But Kaiser also is self-deprecating about his own misperceptions, quoting himself as having written in 1985, when Gorbachev came to power, that the change in leadership “does not mean a transformed Soviet Union. . . . Its political style will remain autocratic and arbitrary.”

Charmingly, he admits to “the ungenerous limits of my own imagination.” He is in good company. When it comes to the Soviet Union, reality outruns fantasy.