On April 21, 1898, the United States declared war against Spain. The causes of the conflict were many, but the immediate ones were America's support of Cuba's ongoing struggle against Spanish rule and the mysterious explosion of the U.S.S. Maine in Havana Harbor. It would be the first overseas war fought by the United States, involving campaigns in both Cuba and the Philippine Islands.
The Spanish fleet guarding the Philippines was defeated by the U.S. Navy under the command of Commodore George Dewey on May 1, 1898. Ignorant of Dewey’s success, President McKinley authorized the assembling of troops in order to mount a campaign against the capital of Manila. The military base best suited as the staging point for troops bound for the Philippines was the Presidio of San Francisco. The majority of these soldiers were volunteers, originating from all over the United States, gathering and training at the Presidio before the long sea voyage to the Philippines and their part in, as Secretary of State John Hay put it, the "splendid little war."