The United States prevailed over Spain in the Spanish–American War of 1898. For a time McKinley resisted growing public sentiment for the U.S. to stop Spain from perpetrating alleged atrocities in nearby Cuba, including hauling civilians into concentration camps. Some leading journalists led the call for McKinley to initiate military action. A cataclysmic turning point occurred with the explosion and subsequent sinking of the U.S.S. Maine, a U.S. warship sent to Havana, Cuba. The circumstances surrounding the destruction of the ship remain cloudy. Whatever the cause, the effect was that McKinley asked Congress for a declaration of war in April 1898.

Congress agreed by a wide margin in the House and a narrow margin in the Senate. U.S. armed forces dominated, notably Theodore Roosevelt and his band of so-called “Rough Riders” who won the Battle of San Juan. The battle earned Roosevelt national hero status and ultimately the vice presidency.

By a peace agreement, the United States gained control of Spanish interests in Guam, Puerto Rico, and the Philippines. Guam and Puerto Rico remain U.S. territories to this day. U.S. occupation in the Philippines led to the Philippine–American War, which lasted several years beginning in 1899. It lasted until the administration of President Theodore Roosevelt.