Final answer:
The sinking of the USS Maine in Havana Harbor was the cause of the Spanish-American War.
Step-by-step explanation:
The cause of the Spanish-American War was The sinking of the USS Maine in Havana Harbor.
In February 1898, the USS Maine exploded and sank in Havana Harbor. While investigations later revealed it was likely an accident due to an internal combustion involving stored fuel, at the time, American journalists sensationalized the event, blamed the Spanish, and called for war.
The sinking of the USS Maine became a rallying cry, and Congress declared war on Spain on April 25, 1898.