Final answer:
The United States declared war on Spain in 1898 due to the explosion of the USS Maine, American economic interests and concerns for Cuban stability and independence, and humanitarian support for the Cuban revolt against Spanish rule.
Step-by-step explanation:
The United States declared war on Spain in 1898 primarily due to the explosion of the USS Maine in Havana harbor, which was blamed on a Spanish mine, and the pressure from the American public and media to 'remember the Maine' and avenge the loss of U.S. lives. Additionally, the U.S. had economic interests in Cuba and was concerned about the stability and independence of the island, which was under Spanish control. The humanitarian plight of the Cubans, who were revolting against Spanish rule, also played a part in swaying American opinion towards intervention. President McKinley ultimately requested a declaration of war from Congress after Spain refused his call to end its repressive policies in Cuba and to declare Cuban independence, leading to the Spanish-American War.