THE LONG PEACE Inquiries Into the History of the Cold War By John Lewis Gaddis Oxford University Press 332 pp. $24.95

PROFESSIONAL diplomatic historians will automatically read this new book by John Lewis Gaddis, Distinguished Professor of History at Ohio University. Though only in his mid-forties, he now qualifies as "dean" of his profession. Policymakers, too, will read The Long Peace, for its author is well known to them -- or to their diplomatic and national security advisers.

The general reader should also find time for this important book. Its shortcomings are minor -- some overlapping of subject matter, inevitable in any work built from separate conference papers, and Gaddis' inflation of the early Cold War role of George F. Kennan, whose biography Gaddis is currently writing.

The Long Peace's strengths, in contrast, are both numerous and important. Without ever drawing attention to himself with any of the fireworks of a "stylist," Gaddis writes superbly well, no mean task when mixing narrative, analysis, personal reflection and advocacy. Though an energetic and imaginative researcher undaunted by days in dusty archives, Gaddis' powers of synthesis are, as ever, most impressive of all.

Constantly in the midst of the international community of scholars writing on the last half century's history -- chunks of this book originate in conferences staged in Manhattan, Washington, Mount Kisco, Palo Alto, Oslo, Beijing, and Noresund, Norway -- Gaddis is not only au courant but always eager to tackle the toughest questions of all. Gaddis' very first book, published just when both America's diplomatic and historiographic consensus had crumbled, provided nothing less than a definitive account of Moscow's and Washington's roles in setting off the Cold War -- The United States and the Origins of the Cold War, 1941-1947 (1972). In Strategies of Containment: A Critical Appraisal of Postwar American National Security Policy (1981), Gaddis analyzed almost two generations of American diplomatic and strategic policy toward the Soviet Union.

In this book Gaddis appears on the surface more modest, at least chronologically, for he mostly concerns himself with the policymaking of the Truman-Eisenhower era. He asks questions that go to the heart of the matter; he offers subtle, skeptical answers clearly open to continuing debate. Why, for example, after 1945, did Soviet actions not only antagonize Americans but also arouse fear among them? Why, though U.S. officials in the 1940s and 1950s never expected Soviet tanks and infantry to march on the West, did Washington nonetheless create a massive strategic counterforce? Why did Americans in the decade after Hiroshima find it so difficult to separate "vital" from "peripheral" interest in Asia? How did U.S. decision-makers decide to "deter" themselves from using nuclear weapons against the Soviet Union in the heyday of American nuclear superiority? Why, though always aware of splits within the "communist" world, did Washington wait until the Nixon-Kissinger era to exploit them effectively? Why have the United States and the Soviet Union since the early '60s accepted and even welcomed the optical scrutiny of each other's space satellites? And why, Gaddis asks in the title chapter, given all the hostility between these two nations, have Americans and Russians never actually come to military blows?

GADDIS DOES not offer simplicities in print, so it would be better for readers to discover for themselves not only his answers but how he has arrived at them. They will discover at work a restless mind and a historian shrewder than most in using work of his cousins in political science. They will find a sober and skeptical view of human nature, as well as Gaddis' Kennan-like belief that "peace" is best maintained through tacit and informal arrangements crafted by superpowers fearful of exposing themselves to an open codification of either mutual fears or understandings.

Though not his express intention, Gaddis joins the growing chorus of writers who believe that the West has already "won" the Cold War. Marxism-Leninism's ideological appeal should no longer worry our policymakers. Communist states themselves are trying to figure out how to inject market techniques into their economies without at the same time undermining the party's monopoly on political rule. Authoritarian states as disparate as Nicaragua and South Korea are buffeted by the winds of a world-wide drive for democracy and individual rights. None of this, Gaddis understands, means that we are out of woods. The Soviet Union remains a potentially dangerous adversary, requiring both vigilance and subtlety on the part of U.S. officials. They should also follow the counsel of George F. Kennan, who remarked in 1949 that we in the United States "must concentrate . . . on our own self-respect: on keeping ourselves and our friends above water amid the genuinely great dangers that modern civilization holds, on exercising as beneficial an influence as we can abroad without claiming that we have the insight or the power to effect any vast change of human institutions on a global scale . . ."

A succinct statement hinting at Gaddis' own philosophy comes in his comment about two generations of nuclear-tipped U.S.-Soviet "peace": "After all, we could do worse."

Robert L. Beisner, professor of history at The American University, is working on a book on the American diplomatic tradition.