In February two events crystallized U.S. opinion in favor of Cuban independence. First, the Spanish minister in Washington, Enrique Dupuy de Lóme, wrote a letter critical of President McKinley that fell into the hands of the Cuban junta in New York. Its publication caused a sensation, but Sagasta quickly recalled Dupuy de Lóme. A few days later, however, the Battleship Maine, which had been sent to Havana to provide a naval presence there exploded and sank, causing the death of 266 sailors. McKinley, strongly opposed to military intervention, ordered an investigation of the sinking as did Spain. The Spanish inquiry decided that an internal explosion had destroyed the vessel, but the American investigation claimed an external source.
The reluctant McKinley was then forced to demand that Spain grant independence to Cuba, but Sagasta refused, fearing that such a concession would destroy the shaky Restoration Monarchy. It faced opposition from various domestic political groups that might exploit the Cuban affair by precipitating revolution at home. Underlying strong Spanish opposition to Cuban freedom was the traditional belief that God had granted Spain its empire, of which Cuba was the principal remaining area, as a reward for the conquest of the Moors. Spanish honor demanded the defense of its overseas possessions, including Puerto Rico and the Philippines. Spain sought diplomatic support from the great powers of Europe, but its long-standing isolation and the strength of the U.S. deterred sympathetic governments from coming to its aid.
On 25 April Congress responded to McKinley's request for armed intervention. Spain had broken diplomatic relations on 23 April. The American declaration of war was predated to 21 April to legitimize certain military operations that had already taken place, particularly a blockade of Havana. To emphasize that its sole motive at the beginning of the struggle was Cuban independence, the U.S. Congress passed a resolution, the Teller Amendment, that foreswore any intention of annexing Cuba.
Neither nation had desired war but both had made preparations as the crisis deepened after the sinking of the Maine. McKinley, having opposed war, hoped to end it quickly at the least possible expenditure of blood and treasure. The U.S. possessed a small well-trained navy, but the army was composed of only twenty-eight thousand regulars. Spain had large garrisons in Cuba and the Philippines, but its navy was poorly maintained and much weaker than that of the U.S. Prewar planning in the U.S. had settled upon a naval blockade of Cuba and an attack on the decrepit Spanish squadron at Manila to achieve command of the sea and preclude reinforcement and resupply of the Spanish overseas forces. These measures would bring immediate pressure on Spain and signal American determination. The small army would conduct raids against Cuba and help sustain the Cuban army until a volunteer army could be mobilized for extensive service in Cuba. Spain was forced to accept the U.S. decision to fight on the periphery of Spanish power where its ability to resist was weakest.
The war began with two American successes. Admiral William Sampson immediately established a blockade of Havana that was soon extended along the north coast of Cuba and eventually to the south side. Sampson then prepared to counter the Spanish effort to send naval assistance. Then, on 1 May, Commodore George Dewey, commanding the Asiatic Squadron, destroyed Admiral Patricio Montoyo's small force of wooden vessels in Manila Bay. Dewey had earlier moved from Japan to Hong Kong to position himself for an attack on the Philippines. When news of this triumph reached Washington, McKinley authorized a modest army expedition to conduct land operations against Manila, a step in keeping with the desire to maintain constant pressure on Spain in the hope of forcing an early end to the war.
On 29 April a Spanish squadron commanded by Admiral Pascual Cervera left European waters for the West Indies to reinforce the Spanish forces in Cuba. Sampson prepared to meet this challenge to American command of the Caribbean Sea. Cervera eventually took his squadron into the harbor at Santiago de Cuba at the opposite end of the island from Havana where the bulk of the Spanish army was concentrated.
As soon as Cervera was blockaded at Santiago (29 May) McKinley made two important decisions. He ordered the regular army, then concentrated at Tampa, to move as quickly as possible to Santiago de Cuba. There it would join with the navy in operations intended to eliminate Cervera's forces. Also on 3 June he secretly informed Spain of his war aims through Great Britain and Austria. Besides independence for Cuba, he indicated a desire to annex Puerto Rico (in lieu of a monetary indemnity) and an island in the Marianas chain in the Pacific Ocean. Also, the United States sought a port in the Philippines but made no mention of further acquisitions there. The American message made it clear that the U.S. would increase its demands, should Spain fail to accept these demands. Sagasta was not yet ready to admit defeat, which ended the initial American attempt to arrange an early peace.