IN ONE OF THE MOST memorable sections of The Wilderness Warrior: Theodore Roosevelt and the Crusade for America (HarperCollins, $35), the 26th president makes his first visit to Yosemite National Park with Sierra Club founder John Muir as his guide. Traveling on horseback in 1903—the East Coast aristocrat Roosevelt playing cowboy, as usual, in a Stetson and jodhpurs; scraggly-bearded Muir "looking rather like a hobo who had been cleaned up for a photo"—the pair spent three days camping and bonding. "Lying out at night under the giant sequoias had been like lying in a temple built by no hand of man," Roosevelt would later write. Within hours of leaving Yosemite, he telegraphed his Interior secretary, Ethan Hitchcock, and ordered protected status for a larger swath of California's great redwood groves.

That's how T.R. rolled: Close encounters with new landscapes drove the Harvard-trained naturalist and fanatical fair-chase hunter to save wildernesses for future generations. And save them he did. While in the Oval Office, from 1901 to 1909, Roose­velt protected some 230 million acres of America's wildlands—five times more than all previous presidents combined. Often ignoring Congress and demands from railroad, oil, and timber bigwigs, he created 150 national forests, five national parks, and 18 national monuments. In this brick-thick, rather reverent biography, Brinkley, a historian and bestselling author (Tour of Duty, The Great Deluge), gives us the most insightful account yet of Roose­velt's evolution from sickly, bird-nest-col­lecting schoolboy to the biggest, baddest conservationist of the 20th century.

Brinkley spends just a paragraph or two on events that might merit full chapters in a typical biography—Roosevelt's 1906 Nobel Peace Prize, for example. Instead the author hones in on (mostly compelling) details about the people and experiences that made T.R. the naturalist tick. At an age when today's kids are still into SpongeBob, young "Teedie" was asking his parents questions about the theory of evolution. Darwin became Roosevelt's guru; as an adult, he often carried The Origin of Species in his saddlebag. But as we might have suspected, the thing that really cemented T.R.'s obsession with conservation was old-fashioned adventure—especially if it involved firearms. Brinkley vividly describes his many exploits, from a "cougar-collecting" trip in the Colorado Rockies to a wolf hunt in Oklahoma, where the energetic president was saddened by the absence of buffalo. He soon launched a reintroduction program.

Of course, Roosevelt's modus operandi was not simply to protect and preserve. His enthusiasm for the "wise use" policies of Forest Service chief Gifford Pinchot irked preservation purists like Muir; his bloodlust has tarnished his conservationist image in some circles; and his push for numerous western reclamation projects, such as Arizona's Theodore Roosevelt Dam, hasn't won him any points with modern enviros. But Brinkley defends Roosevelt's legacy, painting him as a well-meaning, complex character—a dyed-in-the-wool-hunting-breeches Darwinian who truly loved nature but was also hell-bent on conquering it. Did he play up the gun-toting, Wild West persona to offset his aristocratic upbringing? Partly, but so what? T.R. left us with one of the most progressive wilderness-protection systems in the world. Even readers who skim over the particulars of yet another hunting expedition will close this book with a better appreciation for Roose­velt's forward-thinking genius—and, just as satisfying, the history of the American conservation movement in its formative years.