On April 25, 1898, the United States formally declared war on Spain, after the battleship U.S.S. Maine was sunk in Cuba's Havana harbor. By December of 1898, the United States and Spain had signed a peace treaty, whereby Spain gave up it's claims to Cuba, Puerto Rico and Guam, and - for $20 million - transferred the Phillipines to the United States.
U.S. Battleship Maine,
Samuel H. Gottscho, photographer, taken before February 15, 1898.
Coincidently, the Spanish-American War was the first US war in which the motion picture industry played a significant role. If you'd like to see early film presentations on this war, click on The Spanish-American War in Motion Pictures to see the Library of Congress's offerings. There's lots of additional info about the Spanish-American war on this site, so be sure to have your bowl of popcorn ready before you start!