Richard Rhodes' fourth installment of his epic history will be his last, he says, but it does not conclude the subject of the nuclear age.

Twilight of the Bombs begins in 1990 and, with a few flashbacks, carries a worrisome plot line up to the present. In short, since the first bombs fell on Hiroshima and Nagasaki in 1945, nothing quite so bad has happened, but the suspense is killing us.

Like Arsenals of Folly that came before it, Twilight is more of a political account than the magisterial scientific history of the Making of the Atomic Bomb, which won the Pulitzer Prize in 1988, and its sequel, Dark Sun: The Making of the Hydrogen Bomb.

With a perspective fixed on nuclear weapons, Rhodes' soaring and swooping eagle eye has noticed features in the political landscape of the last 20 years that most of us have overlooked.

Few judgments have the authority and clarity Rhodes can bring to bear as he sorts through the aftermath of the age of the superpowers. Even his earlier book on the atomic bomb plays a role as a kind of manual for nuclear weapons inspectors in Iraq.

If the narrative line of this book seems somewhat disorganized compared to the others, it may only be because the subject itself has dispersed. As Stanislav Shushkivich, the chairman of the Belarusian Supreme Soviet predicted, "the nuclear monster could split up into many little monsters."

New Mexico's own nuclear Valhalla in Los Alamos has continued to play a prominent role, but its technical supremacy has faded. As conditions changed, the lab worked in the background on weapons maintenance, intelligence analysis, nuclear safeguards and nonproliferation.

Compared with the early decades of Rhodes' account, LANL sent fewer heroes into the arena.

One of those, as Rhodes relates, was former LANL director Siegfried Hecker, who along with colleagues in the American nuclear complex, developed relationships with counterparts in Russia and began the long process — still under way today — to secure the most threatening remnants of a rival nuclear empire.

The Cold War ended with the fall of the former Soviet Union and amid prodigious heaps of weapons on both sides. Surely the war was over by the time Mikhail Gorbachev resigned and Boris Yeltsin became the president of Russia. The book tells the story of how the subsequent transition was held together in the midst of the wars in Iraq and the advent of new nuclear threats and how our attention switched almost unnoticed from the horrors of mutually assured destruction and nuclear winter to the scary, but far more diminutive, threat of nuclear terrorism.

From "horrorists" to terrorists may be a kind of warped progress, but Rhodes remains at least optimistic enough to believe with Niels Bohr, the Danish physicist, that there is unity in security that will ultimately help the countries of the world conspire to save themselves.

As massive nuclear weapons stockpiles became increasingly useless and expensive to maintain, the weapons have increased in luster. Rhodes repeats in many versions an axiom developed by the Canberra Commission, an Australian initiative that began in 1995 to figure out how to get rid of nuclear weapons. As formulated by Richard Butler, the commission's chairman, the axiom goes, "As long as any state has nuclear weapons, others will seek to acquire them."

The induction of Pakistan, India and North Korea as nuclear powers followed quickly on the heels of the Cold War, and there are signs that others, from Iran to Myanmar, are knocking at the door.

On another level, nuclear proliferation potentially nests inside the fuel that runs the nuclear power plants that are set to take a grip on our energy-starved and carbon-choked planet.

Meanwhile, the weapons stockpiles have dwindled and are scheduled to shrink a bit more, but that has animated countervailing forces.

The fewer the weapons, the more vulnerable certain powerful interests feel. Rhodes calls them "a host of political and military-industry leaders who feared cheating or a decline in government largess or who balked at giving away the long-standing political advantage of threat inflation."

Characteristically, Rhodes sees in his subjects a stubborn streak of self-destructive behavior, tempered by a steady determination on behalf of a world that would ideally be nuclear free.

He finds reassurance in passages of poetic wisdom inspired by the pressurized dilemmas of his times.

Of special resonance is this passage that begins the prologue, by J. Robert Oppenheimer, Los Alamos' first lab director: "It did not take nuclear weapons to make man want peace, a peace that would last. But the atomic bomb was the turn of the screw. It has made the prospect of future war unendurable. It has led us up those last few steps to the mountain pass; and beyond there is a different country."

The book is impeccably produced with 29 black-and-white photographs, scrupulous footnotes, extensive bibliography and index.

Contact Roger Snodgrass at roger.sno@gmail.com.