Final answer:
The explosion of the USS Maine in Havana harbor on February 15, 1898, which killed over 250 American sailors, was the catalyst for the Spanish-American War, fueled by yellow journalism accusing Spain of the attack.
Step-by-step explanation:
The cause of the Spanish-American War in 1898-99 was not the sinking of the Lusitania, the annexation of Hawaii, or the Treaty of Paris, but rather the explosion of the USS Maine. The USS Maine mysteriously exploded in Havana harbor on February 15, 1898, which killed over 250 American sailors.
Although later evidence suggested the explosion was likely an internal accident, at the time, sensationalist journalism, or yellow journalism, pointed fingers at Spain, leading to the war cry 'Remember Maine, to hell with Spain!' This outcry and the public's desire for retribution against Spain spurred the U.S. Congress to declare war on April 25, 1898.