On September 6, 1901, President William McKinley was shot by Leon Czolgosz, an anarchist, while attending the Pan-American Exposition in Buffalo, New York. Eight days later, McKinley died. When news of the assassination reached Republican boss Mark Hanna, he is said to have growled, “Now that damned cowboy is president.” He was speaking of Vice President Theodore Roosevelt, the colorful hero of San Juan Hill, who had indeed cattle ranched in the Dakotas in the 1880s.

Roosevelt immediately reassured the shocked nation that he intended “to continue absolutely unbroken” the policies of McKinley. But Roosevelt was as different from McKinley as the nineteenth century from the twentieth. An activist and a moralist, imbued with the progressive spirit, Roosevelt would turn the Executive Mansion, which he insisted on calling the White House, into a “bully pulpit.” Under his leadership, he achieved major reforms, advocated conservation and antitrust lawsuits, and championed the nation’s emergence as a world power. In the process, Roosevelt would work to shift the nation’s center of power from Wall Street to Washington.

After serving nearly two full terms as president, Roosevelt left office at the height of his powers. Any man would have found it difficult to follow in his footsteps, but his handpicked successor, William Howard Taft, proved poorly suited to the task. Taft’s presidency was marked by vigorous trust-busting but would end with a progressive stalemate and a bitter break with Roosevelt that ultimately caused a schism in the Republican Party.