Book excerpt: It should have been a grand day at the Pan-American Exhibition in Buffalo, New York, in 1901. President William McKinley shook hands with well-wishers who had lined up to meet their popular leader. But one man stepped forward with a pistol hidden under a handkerchief wrapped around his right hand. Two shots rang out, both striking McKinley in the abdomen. As the nation puzzled over the shooter and the ease of his crime, the president suffered for days before finally dying. Vice President Theodore Roosevelt was sworn in as president, becoming the youngest person ever to hold the job. The country -- and the world -- would never be the same.