Final answer:
The sinking of the USS Maine had the greatest effect in contributing to the Spanish-American War.
Step-by-step explanation:
The factor that had the greatest effect in contributing to the Spanish-American War was the sinking of the USS Maine.
On February 15, 1898, the USS Maine, an American battleship, exploded in Havana Harbor, Cuba. Although the cause of the explosion was not definitively determined, the event was blamed on Spain by many in the United States press.
This incident, along with growing tensions between the US and Spain over Spanish colonial rule in Cuba, led to the outbreak of the war in April 1898.
Learn more about cause of the Spanish-American War