The most significant thing about John Lewis Gaddis’s “The Cold War: A New History” (Penguin Press, 333 pages, $27.95) is the fact that it could be written at all. For two generations after 1945, the Cold War was not a subject for historians, but for area studies specialists and international relations theorists, nuclear strategists, and Kremlinologists. There could be no definitive history of the Cold War, for the simple reason that it seemed unlikely ever to end. Yet today’s college students learn about the rivalry between America and the U.S.S.R. in roughly the same spirit of detached curiosity as that of Athens and Sparta. As Mr. Gaddis, a Yale professor and a leading scholar of the subject, writes in his preface, the students in his course on the Cold War find it “not all that different from the Peloponnesian War.”

The comfortable banality of the subject, as Mr. Gaddis recognizes, is the best evidence of the magnitude of America’s triumph. Americans today have no shortage of geopolitical fears and anxieties; in some ways, our post-September 11 world is more immediately threatening because it is more unpredictable than the Cold War. (More Americans have died at the hands of Arab terrorists, after all, than were ever killed by Soviet missiles.) But anyone born before the mid-1970s can still remember the incomparable terror of the Cold War, which those younger than us will hopefully never know: The constant dread of nuclear war, the absurdly hyper-rational strategizing about the apocalypse.

In writing “The Cold War,” Mr. Gaddis has these readers of the future in mind. For 35 years, Mr. Gaddis has been one of the leading writers on the subject; he is the rare scholar who has seen his subject shift from current event to history. In this short book, he boils down all his expertise into a simple, streamlined introduction to the geopolitical events of 1945-89. Proceeding thematically, rather than strictly chronologically, Mr. Gaddis misses some opportunities for dramatic storytelling, as when he skips over the day-by-day progress of the Cuban Missile Crisis; some crucial episodes – especially the Vietnam War – remain out of focus.

But “The Cold War” succeeds in making clear the basic strategic and ideological forces that drove the conflict. The seeds of the U.S.-Soviet rivalry, Mr. Gaddis shows, were already in place well before the end of World War II. Even as the two nations were fighting side by side, Truman wanted to prevent the Soviets from participating in the invasion of Japan, while Stalin alternated between suspicion that the Allies had deliberately postponed opening a second front and regret that D-Day hadn’t been put off still further, thus allowing the Red Army to reach Paris. These latent tensions crystallized into the postwar situation set out in George Kennan’s famous “long telegram,” sent from Moscow to the State Department in February 1946. This document recognized the inherently aggressive and tyrannical nature of communism, and committed the United States to a policy of containing it everywhere.

Mr. Gaddis’s central insight, and the lesson that readers of “The Cold War” will take away when the specifics of Inchon and Able Archer are forgotten, is that the peculiar dynamics of the Cold War inverted traditional assumptions about power – how to get it, how to use it. The United States and U.S.S.R. were so much more dominant than the empires of pre-war Europe that the term “superpower” had to be coined to describe them. Yet two factors new to the Cold War world – the ideological politics of containment and the apocalyptic potential of nuclear weapons – meant that the superpowers were actually sharply constrained in their exercise of power.

Again and again, Mr. Gaddis shows, American and Soviet leaders puzzled over how to translate their theoretical supremacy into actual influence. The very devastation promised by nuclear weapons meant they could not be used to resolve conflicts – as Mr. Gaddis shows in a vivid counterfactual description of what might have happened had the United States used atomic bombs to stop the Chinese advance in Korea. And because both superpowers committed their prestige to supporting their ideological allies at all costs, they repeatedly found themselves fighting battles they would never have chosen.

America went to war in South Korea not because that country had any real strategic importance, but to avoid giving an impression of weakness that might jeopardize its other, more crucial alliances. A similar logic led to the Vietnam War. By committing itself publicly to containment, in other words, America surrendered the initiative to the Soviets, committing itself to respond to every provocation, even when victory might be more costly than defeat. Mr. Gaddis does not discuss whether this was a price worth paying, or a flaw inherent in Kennan’s initial Cold War strategy.

In fact, the greatest weakness of “The Cold War” is his reluctance to interrogate the mistakes, weaknesses, and evils to which the Cold War exposed even its best combatants. There can be no doubt of America’s moral superiority to the Soviet Union in every respect, or that the final American victory was a cause for unmixed rejoicing. But Mr. Gaddis skates too quickly over the shame of McCarthyism, the danger of CIA coups and assassinations, and the arrogance of the technocracy that fought and lost the Vietnam War. His picture of Americans and their government can sound absurdly naive – as when he writes that, in the 1950s, “the idea that their leaders might lie was new to the American people.”

In general, Mr. Gaddis’s quest to explain the Cold War to the uninformed reader leads him to sacrifice a great deal of nuance and depth – finally, perhaps, too much to make his picture trustworthy. In his telling, the Cold War becomes a grand duel fought by a handful of elite leaders, and determined in large part by their personal strengths and weaknesses: Khrushchev’s impulsiveness and Kissinger’s calculation, Gorbachev’s vacillations and Reagan’s convictions. He even judges the participants on their personal appearance and vices – introducing Stalin with a comment on his “scrawny mustache, discolored teeth, pockmarked face,” describing Beria as a “sexual predator.” This doesn’t just substitute caricature for judgment; it invites the speculation of what a differently motivated historian might make of Nixon’s appearance, or Kennedy’s sexual proclivities. Like much else in “The Cold War,” it shows how crucial it is, even for a sage such as Mr. Gaddis, to keep triumph from turning into triumphalism.