The correct answer to this open question is the following.
Although there are no options for this question is the following.
President McKinley could justify entering the war with the sinking of the USS Maine that was anchored in the harbor of Havana, the capital city of the Island of Cuba,
President McKinley tried to stay away from the conflict between Cubans and Spaniards, but there was so much pressure from the American people and the US newspapers that published reports about Mckinley as a weak leader. That is why after that above-mentioned incident, the US Presidente asked the US Congress for a declaration of war, and the Spanish American War started.