Imagine how you’d feel if you woke up one morning and discovered that all the banks in your town had closed. Not only could you not take out any money, but all your savings had vanished forever.

It happened to millions of Americans during the Great Depression. By 1932, more than 25 percent of the nation was unemployed and full-time jobs were scarce. Many took to hiding their remaining money in their mattresses. Panic was in the air.

In many other countries, this would have set the stage for a revolution and a fascist takeover. Jonathan Alter’s “”The Defining Moment: FDR’s Hundred Days and the Triumph of Hope”” tells a different story. It’s about how the United States slid right to the brink of catastrophe and was pulled back by one of its greatest leaders.

Alter convincingly argues that while FDR may not have stopped the Depression in its tracks – it officially lasted until 1938 – in his first 100 days as president, he saved the country from falling apart.

Alter does not try to cover up FDR’s mistakes; he offers a critical but basically admiring portrait. He notes FDR was no intellectual, but a clever politician who worked by instinct and experimentation.

When FDR was unexpectedly nominated to run against the dour Herbert Hoover in 1932, he beat the despised president easily. What would he do to solve the crisis? Many thought the answer was fascism.

“”If ever this country needed a Mussolini, it needs one now,”” one senator declared. But the new president had no intention of doing that.

Instead, telling America that it had nothing to fear but “”fear itself,”” he set out to revive the economy and save his country from disaster.

The problem with “”The Defining Moment”” is that it takes Alter more than half the book to get to FDR’s hundred days. He spends so much time on FDR’s upbringing and early career that many readers will feel impatient. He also chops up the narrative into a series of chapters so short that some of them feel like magazine articles.

Alter also alternates uneasily between covering his subject’s public and personal life. It seems superfluous for him to tell us, as he does, that FDR’s condition did not prevent him from having affairs. Did that affect his leadership?

If Roosevelt’s first hundred days were indeed the most important first hundred days of any president, as Alter argues, don’t they deserve more than half a book?

Luckily, Alter hits his stride once he gets into FDR’s presidency. His account of how FDR was able to reopen most of the nation’s banks, get starving farmers back on their feet again and give jobs to the unemployed is brisk, readable and enlightening.

“”The Defining Moment”” is a reminder that great leadership can come from the most unexpected men, and surprise even the most jaded cynics.