It was a 9/11 moment, one of those events that are seared indelibly into the memory, much like that steamy August afternoon in 2007 when the Interstate 35W bridge collapsed.

The demise of the Soviet Union was one of those moments. A regime built on a mountain of corpses, in historian Robert Conquest's memorable words, a military superpower that had locked the United States in a potentially lethal minuet for 50 years, simply vanished like wisps of smoke.

In Conor O'Clery's gripping account of the Soviet Union's final day, there is no end to ironic symmetry: The idea that took shape in 1916 in a cold-water flat in Zurich was to be undone by a quiet conversation "held in the backyard of a farm in Amherstburg, Ontario" in May 1983.

That discussion -- between Mikhail Gorbachev, the new Soviet leader on a state visit to Canada, and Alexander Yakovlev, the diplomat who had been banished to Ottawa for speaking the truth in party circles about the abject failures of the system -- would lead to perestroika, Gorbachev's plan for restructuring the moribund Soviet economy, and glasnost, the openness that he hoped would quash endemic corruption and breathe new life into Soviet society.

It didn't work, of course, for it enraged the Communist Party mandarins and led to the August 1991 coup attempt against Gorbachev. Again the irony, for it was not Gorbachev's allies, but his bitter opponent, Boris Yeltsin, who defied the tanks, broke the back of the revolt and released Gorbachev from confinement.

O'Clery, the Irish Times' award-winning correspondent in Russia, portrays the final day as the culmination of a fierce struggle between these two titans. Gorbachev, the intelligent, sophisticated narcissist (Exhibit A: the three-story, custom-built mansion equipped with the latest Western appliances that Gorbachev ordered while the economy bottomed out and ordinary Russians queued for hours at filthy stores that were empty of even basic necessities) vs. Yeltsin, the brash, vulgar, impetuous and enormously savvy politician. One sought to save the Soviet state by reforming it, the other viewed it with utter contempt and strove to bring it down.

In the end, Gorbachev left his residence in tears, Yeltsin outlawed the Communist Party, and Russia was launched on a tumultuous path to a free-market economy and a ramshackle democracy. Vladimir Putin would later call it a "geopolitical catastrophe," and Gorbachev, O'Clery points out, has turned into something of a scold, but one whose warnings are justified.

Here are the personalities, the drama, the betrayals, the bickering and maneuvering, the threats and entreaties behind an event that virtually no one in the West saw coming. Told with authority and narrative grace, O'Clery's book provides a keen understanding and unique perspective on what was one of the most important events in world history.

Michael J. Bonafield, who covered the August 1991 coup attempt in Moscow, writes from Apple Valley.