Theodore Roosevelt became President of the United States at age 42, the youngest person ever to do so, eight days after William McKinley was fatally wounded in Buffalo, New York.

Theodore Roosevelt Jr., also known as Teddy or T. R., was the 26th president of the United States, serving from 1901 to 1909. Roosevelt previously was involved in New York politics, including serving as the state's 33rd governor for two years. He served as the 25th vice president under President William McKinley for six months in 1901, assuming the presidency after McKinley's assassination. As president, Roosevelt emerged as a leader of the Republican Party and became a driving force for anti-trust and Progressive Era policies.

The president of the United States (POTUS) is the head of state and head of government of the United States. The president directs the executive branch of the federal government and is the commander-in-chief of the United States Armed Forces.

William McKinley was the 25th president of the United States, serving from 1897 until his assassination in 1901. A member of the Republican Party, he led a realignment that made Republicans largely dominant in the industrial states and nationwide for decades. McKinley successfully led the U.S. in the Spanish–American War and oversaw a period of American expansionism, with the annexations of Hawaii, Puerto Rico, Guam, the Philippines, and American Samoa.

William McKinley, the 25th president of the United States, was shot on the grounds of the Pan-American Exposition in the Temple of Music in Buffalo, New York, on September 6, 1901, six months into his second term. He was shaking hands with the public when an anarchist, Leon Czolgosz, shot him twice in the abdomen. McKinley died on September 14 of gangrene caused by the wounds. He was the third American president to be assassinated, following Abraham Lincoln in 1865 and James A. Garfield in 1881.