BY HIS OWN RULES: The Ambitions, Successes, and Ultimate Failures of Donald Rumsfeld. By Bradley Graham.

"The blizzard is over!" Donald Rumsfeld declared in the last of some 20,000 memos — or "snowflakes" — that had become a hallmark of his contentious tenure as secretary of defense. During the summer of 2003, a squall of snowflakes and counter-snowflakes blew through the offices of Rumsfeld and Gen. John Abizaid, the newly appointed head of U.S. Central Command, about the definitions of "insurgent" and "guerrilla warfare."

Rumsfeld, over Abizaid's objections, resisted acknowledging the enemy in Iraq as an organized force because doing so would suggest that the U.S. presence there was likely to be long and costly. But his denial merely delayed the inevitable, and, as in a real snowstorm, the cleanup began only after the last flake fell.

Rumsfeld is not a simple man. But the two biggest questions about his tenure at the Pentagon — why the United States invaded Iraq, and why it so bungled the aftermath of the Hussein regime's fall — are often answered with only the simplest of explanations: ideology and hubris.

In this meticulously researched and compelling book, Washington Post reporter Bradley Graham acknowledges these contributors to the national security travails of the Bush years, but he highlights another as well: the secretary of defense's unwavering commitment to military transformation, his vision of a leaner, more lethal Department of Defense. The early phases of the war in Afghanistan apparently vindicated this concept, while the prospect of war in Iraq promised a wider proving ground for it — but the nasty counterinsurgency campaign that followed threatened to undermine it.

Having served briefly as Gerald Ford's secretary of defense, Rumsfeld re-entered the Pentagon in 2001 expecting not to lead troops in war, but to fight the bureaucracy to transform the way the United States waged war. He initially appeared to relish his new role as secretary of war. As often as once a day, he conducted televised press briefings with virtuoso skill, weaving blunt talk with folksy charm. When asked one Sunday whether the United States was close to catching Osama bin Laden, he answered with a characteristic rhetorical question: "If you're chasing a chicken around the barnyard, are you close or are you not close until you get him?"

President Bush nicknamed him "Rumstud." Graham shows that Rumsfeld's commitment to his transformation agenda never waned despite the operational burdens of two major wars.