The U.S. believed that the Spanish had sunk one of their ships, the U.S.S. Maine, off of the coast of Havana, Cuba (one of Spain's territories at the time). After being investigated, no conclusion was drawn as to what or who caused the explosion. However, yellow journalism, written by the infamous William Randolph Hearst and Joseph Pulitzer, said that Spain was the one who sank the ship. This event caused the beginning of the Spanish-American war. The aftermath was Spain's defeat and the gain of territories such as Cuba and the Philippines.
Some people also believe that the US sunk its own ship due to the nature of the explosion in order to start a war with Spain.