The reason why United States declared war against Spain in 1898 was the sinking of U.S. Battleship named Maine which arrived at Havana harbor with an aim to protect the interest of the Americans and to show Spanish some force. However, the ship sank and the Spanish were blamed for this. Thus, the United States declared war on Spain in 1898. This war was justified by the United States saying that they went to the war to free the Cubans from the rule of the Spaniards. It was their duty to help Cuba come out of the Spanish control.