Cuba had long been a colony of Spain, with almost its entire economy based on sugar production. As the second half of the 19th century wore on, many people in Cuba, which had been a Spanish colony, became dissatisfied with the ruling Spanish regime. The Spanish government was riddled with corruption, inefficient, and unwilling to grant the Cuban population any concessions.
Despite obvious Cuban dissatisfaction, the Spanish authorities refused to grant any amount of self-government to the Cubans. As a result, Cuban Nationalists, who wanted to end Spanish rule, fought the Ten Years' War against the Spaniards from 1868-1878. The rebellion finally petered out, though the dissatisfaction motivating the fighting did not disappear. After the war, the Spanish promised reforms, but the Nationalists considered this too little, too late.
When a Nationalist-initiated conflict broke out again in Cuba in 1895, the Spanish, remembering the lengthy Ten Years' War, sent 200,000 troops to Cuba. The Cuban insurrectos responded by wrecking Spanish property in hopes that the Spanish would leave, or at least hoping for US intervention (since the US had significant economic investment in Cuba). The insurrectos directed their destructive rampage at both sugar mills and sugar fields.
In 1896, the Spanish sent the infamous General Weyler, known as "The Butcher," to Cuba to put down the insurrection. Weyler lived up to his name. To prevent the insurrectos from leading the population against Spanish rule, Weyler built concentration camps in which he imprisoned a large portion of the population. Under the harsh and unsanitary conditions in the concentration camps, Cuban prisoners died rapidly, especially from disease.
Segments of the US public, outraged by reports of atrocities in Cuba, immediately cried out for action. President Grover Cleveland (1893-1897), however, was dead set against going to war. He issued an ultimatum: even if Congress passed a declaration of war, he vowed as commander-in-chief of the army to never send the military to Cuba.
The Cuban Nationalists moved against Spain partly because they thought the US likely to come to their aid. The US was investing increasing amounts of money into Cuban sugar production ($50 million by 1895) and conducted a trade with Cuba worth $100 million annually. From the 1860s on, the US had even tried to purchase Cuba from Spain several times.