Defining the root cause of the Spanish-American war or any war can be a subjective matter, depending on from which side of the conflict one views events and why. The war officially started on 23 April 1898, when Spain declared war on the United States in response to a US Navy blockade of Cuba that began on 21 April 1898. The US responded in kind with a declaration of war against Spain on 25 April 1898.
Historically, the main cause that led up to the confrontation was a desire for Cuban independence, involving a decade-long revolt in Cuba against Spanish rule that had strong US sympathies. Popular opinion in the US turned to the idea of war against Spain, however, after the battleship U.S.S. Maine was sunk in Havana harbor on 15 February 1898. Spain was blamed for sabotaging the ship by placing a mine near it that ignited its powder magazines, causing it to sink and resulting in the death of 266 US sailors.
Perhaps the definitive root cause of the Spanish-American war, as well as many other colonial conflicts in the Americas, Africa, Asia and elsewhere, can be traced back to origins in the mid-15th century. During this period, papal decrees from the Vatican by sitting popes gave the right to certain European kings to conquer and convert various land areas as a function of divine right. In 1493, the Spanish-born pope Alexander VI began to decree lands west of the European mainland to Spain, giving it the impetus to be the first nation to sail across the Atlantic Ocean and colonize the Western Hemisphere. By 1825, Spain had lost most of its colonial territory to rival European nations, the United States, and Mexico. Its sole remaining colonies were Cuba, Puerto Rico, and island chains, such as the Philippines.
Cuba watched the liberation of other Latin American countries from Spanish rule, and, from 1868 to 1878, began its own revolt, which came to be known as the Ten Year's War. It was initiated by Carlos Manuel de Cespedes, a Cuban landowner, who, on 10 October 1868, issued a proclamation of Cuban independence known as Grito de Yara, or literally "Cry of Yara," for the small town of Yara along the southern coast of Cuba. The Ten Year's War was the beginning of three wars of liberation that Cubans fought against Spain, leading up to the current uneasy state of affairs in 1898.
Another notable contributing cause of the Spanish-American war was the way in which US foreign policy was shaped by a book entitled The Influence of Sea Power Upon History, 1600-1783, written by Alfred T. Mahan, and published in 1890. Mahan was a US Navy flag officer who advocated in his book that the United States should seize control of Hawaii, the Philippines, and the islands of the Caribbean. He saw them as key locations for military bases to protect US commercial interests in the region.
The war lasted only until 12 August 1898, due in large part to overwhelming American superiority in numbers. Demoralizing losses for Spain also occurred rapidly, such as the US Navy defeating the Spanish Navy in Manila Bay in the Philippines in only six hours time by sinking the entire Spanish squadron of ships. With the cessation of conflict by the signing of the Treaty of Paris, Cuba was granted independence, and Puerto Rico and Guam became US possessions. The Philippines passed from Spanish to US control by the US purchasing them from Spain for $20,000,000 US Dollars (USD). This was a shocking end to Spain's over 400-year-plus process of empire building, and issued in a new era of serious questioning of colonial ambitions worldwide.