It was characteristic of theodore Roosevelt, the fastest-moving of all our presidents, that when photographers tried to get a shot of him departing for Africa in March 1909, the best they could do was capture a blur in front of an Abercrombie & Fitch sign on the Hoboken, New Jersey, waterfront. Still only 50 years old after two terms in the White House, TR had enough pent-up energy to spend 11 months hunting big game in the highlands of Kenya and Uganda. If he survived (Wall Street victims of his regulatory wrath were hoping that the first lion he met “would do its duty”), he intended to march north from Kampala in the new year of 1910, sail down the Nile to Cairo, then embark on a Grand Tour of Europe, not returning home until more than a year had passed.

A much less known photograph was taken of him after his arrival in Nairobi (riding on the cowcatcher of an enormous locomotive of the Uganda Railway). It showed him grinning toothily in the shade of a pith helmet, which was considered obligatory in those days to protect white men from the lethal rays of the equatorial sun. As snapshots go, it wasn’t much, but for some reason when I saw it as a Kenya schoolboy, 40 years later, it registered so vividly that I can see it right now in my mind’s eye. It was printed in a commemorative city booklet entitled Nairobi: The First Fifty Years, and the caption made some reference to the great American president who had visited Kenya, long before I was born, and stayed with Sir Northrop and Lady Macmillan before marching off into the bundu with his rifle and porters in tow.

As it happened, Lady Macmillan was still alive in 1950, a recluse whose rambling stone villa lay just across the river from our house. It was walled off by a dense bamboo forest and almost inaccessible except to a small boy who could snake between the tight rods and peek across the lawn to her veranda—where sometimes I would see the old lady having tea, waited on by servants in white kanzus. It occurred to me then that this must have been the house where the man in the pith helmet had stayed in 1909—and it occurs to me now, knowing TR’s penchant for writing on porches (with an indelible pencil pressing down on a triple-carbon manuscript pad), that the opening chapter of his book African Game Trails may well have been scribbled in that bougainvillea-hung retreat.

The seeds of a biographical relationship sprout unpredictably, and often after long dormancy. I don’t suppose I thought of TR again until 1974, when Richard Nixon, in tears, quoted him just after resigning the presidency. Something about Nixon’s emotion made me remember that old photograph of the grinning hunter, and now here I am, publishing the third (and definitely the last!) volume of a life of one of the most cosmopolitan personalities in our history.

One of the things that made TR “polygonal”—in a friend’s phrase—was his phenomenal knowledge of the world, the legacy of a lifetime of obsessive travel. The Grand Tour he took in the spring of 1910 (during which kings and emperors vied for the privilege of putting him up in their palaces) was, almost incredibly, his fifth. Before he was in his mid teens, he had completed two of them, each more than a year long, and also spent five months in Dresden living with a German family and saturating himself with the German language and culture. (It was there, incidentally, that a certain Fräulein Anna Minkowitz became the first to predict that young “Teedie” would one day be president of the United States.) These tours, organized by his sophisticated father, took in not only the predictable major cities that wealthy Americans visited in the late 19th century but also such far-flung places as County Cork, Ireland, the Dardanelles, and the deserts of Lebanon and Syria. He first saw Jerusalem from the saddle of an Arabian pony.

TR married twice and, on each occasion, went on mammoth European honeymoons. During the first, he took time off from nuzzling the cute Alice Lee Roosevelt (“Our happiness,” he sternly informed me in his diary, “is too sacred to be written about”) to climb the Matterhorn. On the second, he and Edith Kermit Roosevelt took shelter from a torrential rainstorm in a pensione on the Italian Riviera. They sent down for room service and preserved the receipt, which listed among other things an omelet, a bottle of wine, and a bundle of logs to build a cozy fire. My wife, who wrote a biography of Edith, was curious as to why they should set such store by this receipt. She counted ahead nine months, and arrived at the natal day of Theodore Roosevelt Jr.

What distinguished TR as a traveler, in an age when (even then) Americans sought the company of other Americans abroad for security and self-congratulation, was his compulsion to come to grips with every country he entered—not only physically but intellectually and socially, as well. It was not enough to gaze up at Vesuvius from a comfortable carriage: he had to hang over the edge of the crater and cough in its fumes. He wanted to hunt foxes with the British, quiz Hungarians on Magyar history, and eat elephant hearts with Africans. Crossing the Atlantic on a German liner, he went down from first class into steerage and participated in a hot, dark Catholic mass with a crowd of Polish immigrants. On a visit to the mosque of Al-Azhar, in Cairo, he delighted the mullahs by asking to see a medieval Arab scroll (appropriately enough, The Travels of Ibn Battuta). He could not read its calligraphy, but he recited a passage from it anyway, saying that he had memorized a French translation many years before. As colonel of “Roosevelt’s Rough Riders” in Cuba during the Spanish-American War, he swam across Santiago Bay to inspect Morro Castle, undeterred by an escort of sharks: “They [stroke] won’t [stroke] bite [stroke].”

TR was, consequently, not only the youngest president we’ve ever had (being only 42 at the time of his elevation in 1901) but also the best-traveled—at least up to modern times. Indeed, I’m not sure if any of his successors have seen as much of the world as he did, until they got free access to Air Force One. Nor did he ever lose his yearning for other cultures, other climes. Three weeks after quitting the White House in 1909, he was off to Africa. And the most legendary of his travels was yet to come—his exploration of an unknown tributary of the Amazon in 1914. Longer than the Rhine, that “river of doubt” is now known as the Rio Roosevelt.

“It was my last chance to be a boy,” he said after nearly dying on the expedition. I think his reasons for undertaking it were more complex. By then, TR had had his fill of red-carpet touring (“If I see another king, I think I shall bite him.”). As a passionate nature lover and conservationist, he was consumed with a sense that the world’s wildernesses were being carved away. He no longer believed that civilization improved by expanding. On the contrary, it coarsened as it spread: he found his own gentlemanly sanctuary at Cove Neck, in Oyster Bay, Long Island, invaded by a new species, the “moving-picture man of vast wealth.” So he sought a few months of wild freedom in the Brazilian rain forest, only to be struck down by a combination of malaria, abscesses, and heart disease. The pathogens he brought home with him were largely responsible for shortening his life span to only 60 years. But “Theodore the Sudden” never stopped moving, or trying to move, until his final hospitalization on the day World War I came to an end. He died, to international grief, on January 6, 1919.