Final answer:
The explosion of the U.S.S. Maine in Havana Harbor.
Step-by-step explanation:
The leading cause of the Spanish American War was the explosion of the U.S.S. Maine in Havana Harbor. On February 15, 1898, the U.S.S. Maine, an American battleship, exploded and sank in Havana Harbor, Cuba. Although the cause of the explosion remains uncertain, it was widely reported in U.S. newspapers as a result of sabotage by the Spanish, fueling anti-Spanish sentiments and eventually leading to the war.
Learn more about Cause of the Spanish American War