During the Spanish-American War, U.S. forces launch their invasion of Puerto Rico, the 108-mile-long, 40-mile-wide island that was one of Spain’s two principal possessions in the Caribbean. With little resistance and only seven deaths, U.S. troops under General Nelson A. Miles were able to secure the island by mid-August. After the signing of an armistice with Spain, American troops raised the U.S. flag over the island, formalizing U.S. authority over its one million inhabitants. In December, the Treaty of Paris was signed, ending the Spanish-American War and officially approving the cession of Puerto Rico to the United States.

READ MORE: Puerto Rico's Complicated History with the United States

In the first three decades of its rule, the U.S. government made efforts to Americanize its new possession, including granting full U.S. citizenship to Puerto Ricans in 1917 and considering a measure that would make English the island’s official language. However, during the 1930s, a nationalist movement led by the Popular Democratic Party won wide support across the island, and further U.S. assimilation was successfully opposed. Beginning in 1948, Puerto Ricans could elect their own governor, and in 1952 the U.S. Congress approved a new Puerto Rican constitution that made the island an autonomous U.S. commonwealth, with its citizens retaining American citizenship. The constitution was formally adopted by Puerto Rico on July 25, 1952, the 54th anniversary of the U.S. invasion.