THE MEMORY HOLE created by our 24-hour news cycle is vast and deep, and nothing suffers because of it quite like our understanding of the war in Iraq. It is useful to remember that the Bush administration sold this war as a matter of national security and WMDs. If we waited too long, Bush said, our smoking gun might just come in the shape of a mushroom cloud. As we now know, this kind of talk was mostly marketing. The Bush administration planned on removing Saddam long before it took office; it simply needed a reason that would galvanize the American public.

New Yorker writer George Packer accepts this bait-and-switch job as politics as usual, but he is curious about how an administration that campaigned on a platform of isolationism became a firm believer in the most far-reaching foreign-policy adventure in decades. In The Assassins' Gate, Packer tracks this evolution, emerging with a sobering tale about the danger of big ideas. Like several journalists before him, Packer finds the real smoking gun of this war not in WMDs or even 9/11, but in a 13-year-old policy paper called "Defense Planning Guidance of 1992," which was commissioned under Bush I by then-Secretary of Defense Dick Cheney. The paper argued that in the wake of communism's demise, America's top national security goal ought to be "precluding the re-emergence of a new rival." Regime change in Iraq was the centerpiece of this vision.

After 9/11, this paper became the blueprint for America's involvement in the war on terror, opening up, as Packer shows, a window of opportunity for players who wanted to put pet ideas into motion. For instance, Secretary of Defense Donald Rumsfeld looked to Iraq as a larger-scale testing ground for his new, leaner army, which had just won a stunning if short-lived success in Afghanistan.

But Rumsfeld was so wedded to his vision of a 21st-century fighting force that when his own army chief of staff, Anthony Shinseki, estimated that he would need "several hundred thousand" troops for the job in Iraq, Rumsfeld demoted and then effectively fired the four-star general.

In fact, the most alarming part of the book is how often this administration, when faced with news it didn't want to hear, refused to listen and then reprimanded (or even fired) the people who spoke up. As a result, Packer reveals, the United States was sorely unprepared for the realities of life in Baghdad after Saddam was toppled. This led to staggering incompetence in the field, which Packer documents in a series of vivid dispatches from Baghdad. After the city fell, a 25-year-old was put in charge of overseeing "the creation of the Baghdad stock market," writes Packer, "and another 25-year-old ... helped write the interim constitution while filling out his law school applications."

Meanwhile, the Iraqi people's patience was winding down. Power was sporadic, sewers weren't working and revenge killings against former Baath party members were common. Tolerance for this chaos ran out on the watch of L. Paul Bremer III, the second head of the Coalition Provisional Authority. Packer says Bremer made the key failures of not securing the city from looting, of disbanding the Iraqi army and of disbursing funds too slowly. Packer does not attempt to present these failures in the context of Baghdad's history. Sensing, perhaps, his limits as a Western reporter working through translators, Packer keeps his eye fixed on America's shifting presence in Iraq.

But he does find room to note that Americans are not the only ones who fell prey to big ideas. Packer manages to talk to dozens of Iraqis—more often than not they are still hopeful. Several of them even waited out the years of Saddam's power in exile, harboring wild dreams of a new beginning.

Their presence just goes to show that no one in this war has relinquished their plans easily. But the failure of optimistic views—be they Iraqi or American—has led to the rise of a far darker and more dangerous belief held by a growing part of the Iraqi population. And that is resistance at all costs. In the end, Packer notes, that might just prove the most unbending idea of them all.

The Assassins' Gate: America in Iraq by George Packer; Farrar, Straus and Giroux; 467 pages; $26 cloth.