In the late 19th century, the United States confronted rising tensions with Spain primarily over the plight of Cuba. The Cuban struggle for independence from Spanish colonial rule garnered significant sympathy within the U.S., which saw the potential for economic and strategic expansion. Events escalated dramatically following the explosion of the USS Maine on February 15, 1898, in Havana harbor, which resulted in the deaths of 266 American sailors. While the cause of the explosion remained controversial, the incident fueled anti-Spanish sentiment and calls for military intervention. The U.S. public, stirred by yellow journalism from major newspapers, rallied behind the idea of aiding the Cuban revolutionaries and punishing Spain.

On April 21, 1898, the U.S. initiated a blockade of Cuba, signifying a critical shift in its military posture. This blockade was seen as an act of war, and on April 25, Congress acted swiftly, declaring that a state of war had existed since the blockade began. This declaration came just days after President William McKinley had communicated with Congress regarding the situation in Cuba, emphasizing humanitarian concerns and the need to protect American interests.

The declaration marked the formal entry of the U.S. into the Spanish–American War, leading to rapid military actions in both the Caribbean and the Pacific, and resulted in significant territorial gains for the U.S. following its victory over Spain. The war showcased the United States' emerging military strength and marked a transition towards a more interventionist foreign policy, laying the groundwork for its status as a global power in the 20th century.