In his early 20's, Theodore Roosevelt divided his life between politics and writing. He served three one-year terms in the New York Assembly, where he supported civil service reform, legislation benefiting working people, and bills to improve the New York City government. Following two years in the Dakotas as a rancher, he returned in 1886 to run for New York City mayor. He finished third in a three-way race. He continued his writing, with three major works completed by the end of the decade.

In 1889, President Benjamin Harrison named Roosevelt civil-service commissioner. He left in 1895 to become president of New York City's Board of Police Commissioners, leaving two years later to become assistant secretary of the navy in the McKinley administration. A nationalist and expansionist, he used his office to prepare the nation for war with Spain. With the onset of the Spanish-American War, he helped to organize the Rough Riders. He saw considerable action in Cuba, returning to this country as a colonel.

Roosevelt's fame propelled him to the forefront of New York politics and he won a close election for governor of the state in 1898. In that office he championed civil service and approved bills supportive of labor and social reform. As McKinley's running mate in 1900, the assassination of the president in late 1901 moved Roosevelt to the presidency. The following year he brought action under the Sherman Anti-Trust Act against Northern Securities Company, a railroad monopoly of the northwest. The suit began a "trust-busting" crusade against big business that carried over into his second administration.

When the anthracite coal miners struck in 1902, Roosevelt became the first president to intervene in a labor-management dispute. He threatened to seize the mines to persuade owners to accept mediation. A proponent of conservation, he backed Gifford Pinchot in expanding the nation's forest reserve, setting aside millions of acres of coal lands and encouraging conservation on the state level. On the international scene, he was even more bold than he was domestically. After Colombia rejected a treaty in 1903 giving the United States rights to a canal across the isthmus of Panama, he supported a Panamanian revolt and then negotiated a similar treaty with the new nation. He then supervised the construction of the Panama Canal.