On the night of 15 February l898 the battleship USS Maine was shattered by an explosion which sent the ship and two-thirds of her crew to the bottom of Havana harbor. Bolstered by wide-spread sympathy for those who were seeking Cuban independence from Spain's colonial rule, the emotional Maine tragedy forced the already strained Spanish-American relations to the breaking point, precipitating a short war rapidly decided by two naval engagements.
On April 19 Congress passed a joint resolution proclaiming Cuba "free and independent", and when signed by McKinley the next day amounted to a declaration of war.
The first military action of the war was the battle for Manila in the Philippines. At the eve of the war, a group of six ships under the command of Commodore George Dewey were in Hong Kong, and they immediately departed for the Spanish possession of the Philippines. The Spanish fleet and the batteries surrounding Manila were destroyed on May 1
without a single U.S. casualty. The conquest of Manila was as much a political as a military advancement.
Available http://www.loc.gov/rr/hispanic/1898/intro.html