The Puerto Rico campaign was the American military sea and land operation in Puerto Rico during the Spanish–American War, which resulted in the invasion, occupation, and annexation of the archipelago and island by the United States, and the cession of said territory by Spain. The offensive began on May 12, 1898, when the United States Navy attacked the capital, San Juan. Though the damage inflicted on the city was minimal, the Americans were able to establish a blockade in the city's harbor, San Juan Bay. On June 22, the cruiser Isabel II and the destroyer Terror delivered a Spanish counterattack, but were unable to break the blockade and Terror was damaged.

The land offensive began on July 25, when 1,300 infantry soldiers led by Major General Nelson A. Miles disembarked off the coast of Guánica. After winning the first skirmish, the Americans advanced to Coamo, where they engaged Puerto Rican and Spanish troops in battle. With two defenders and four Americans dead, the defenders retreated. The United States forces were able to seize control of Fajardo on August 1, but were forced to withdraw on August 5 after a group of 200 Puerto Rican–Spanish soldiers led by Pedro del Pino counterattacked, while most civilian inhabitants fled to the nearby lighthouse. The Americans encountered more opposition as they advanced towards the main island's interior. They engaged in two crossfires in Guamaní River and Coamo, both of which were inconclusive as the allied soldiers retreated. A battle in San Germán concluded in a similar fashion with the Spanish retreating to Lares.

On August 9, 1898, American troops that were pursuing units retreating from Coamo and Asomante encountered heavy resistance in Aibonito and retreated after six of their soldiers were injured. They returned three days later, reinforced with artillery units and attempted a surprise attack. After about an hour of fighting, Spanish artillery batteries had been silenced. American guns advanced some 2,150 yards and set up positions, but soldiers reported seeing Spanish reinforcements nearby and the guns were withdrawn back to the main line. Shortly before the launch of a flanking movement on the Spanish, all military actions in Puerto Rico were suspended on August 13, after U.S. President William McKinley and French ambassador Jules Cambon, acting on behalf of the Spanish government, signed an armistice whereby Spain relinquished its sovereignty over the territories of Puerto Rico, Cuba, the Philippines and Guam.

In 1890, Captain Alfred Thayer Mahan, a member of the Navy War Board and leading U.S. strategic thinker, wrote a book titled The Influence of Sea Power upon History in which he argued for the creation of a large and powerful navy modeled after the Royal Navy. Part of his strategy called for the acquisition of colonies in the Caribbean Sea that would serve as coaling and naval stations, being strategic points of defense upon the construction of an isthmusian canal.

This idea was not new; William H. Seward, a former Secretary of State for various Presidents, among them Abraham Lincoln and Ulysses Grant, had strongly advocated that a canal be built either in Honduras, Nicaragua or Panama, and that the United States annex the Dominican Republic and purchase Puerto Rico and Cuba. The idea of annexing the Dominican Republic failed to receive the approval of the U.S. Senate, and Spain did not accept the 160 million dollars which the U.S. offered for Puerto Rico and Cuba. Mahan made the following statement to the War Department: "Having therefore no foreign establishments either colonial or military, the ships of war of the United States, in war will be like land birds, unable to fly far from their own shores. To provide resting places for them where they can coal and repair, would be one of the first duties of a government proposing to itself the development of the power of the nation at sea."

Since 1894, the Naval War College had been formulating plans for war with Spain. By 1896, the Office of Naval Intelligence had prepared a plan which included military operations in Puerto Rican waters. Not only was Puerto Rico considered valuable as a naval station, Puerto Rico and Cuba were also abundant in a valuable commercial commodity which the United States lacked: cane sugar.

On February 15, 1898 USS Maine exploded and sank in Havana Harbor, Cuba. According to the Navy's leading weapons expert, Philip Alger, the explosion was due to a coal fire igniting a reserve magazine of six tons of gunpowder, much of which was already degrading due to the humid climate. However, the United States forwarded an ultimatum to Spain to withdraw from Cuba following the sinking of Maine. In response, Spain broke off diplomatic relations with the United States, and on April 23, 1898, Spain declared war. On April 25, the U.S. Congress declared that a state of war between the United States and Spain had existed since April 20. One of the United States' principal objectives in the Spanish–American War was to take control of Spanish possessions in the Atlantic—Puerto Rico and Cuba—and their possessions in the Pacific—the Philippines and Guam.

On April 27, U.S. ships, the monitor USS Puritan, and the armored cruisers USS New York, and USS Cincinnati, bombarded the Spanish fortifications at Matanzas Bay in Cuba. By July 16, an armistice was signed at the Arbol de La Paz (a large ceiba tree) in Santiago de Cuba by U.S. and Spanish forces ending hostilities in Cuba and its waters. The United States then directed its undivided military resources to Puerto Rico. Two leaders of the Puerto Rican section of the Cuban Revolutionary Party, Julio J. Henna and Roberto H. Todd, had written to U.S. President McKinley asking that Puerto Rico be included in whatever intervention was planned for Cuba as early as March 10. They even provided the U.S. government with information about the Spanish military presence on the island.