Puerto Rico was ceded to the United States in a treaty ending this:
The Mexican-American War
The Mexican Revolution
The Spanish-American War
The American Civil War
World War I
The Spanish-American War. Puerto Rico was ceded to the United States in the Treaty of Paris, ending the Spanish-American War. The treaty was signed by the United States and Spain on December 10, 1898 (the U.S. Senate ratified the treaty February 6, 1899). The U.S. military governed the island from October 1898 until May 1900, when the U.S. Congress instituted a civil government. The civil government underwent many changes until a Constitutional Assembly formed in 1950 and established a Commonwealth of Puerto Rico, which was proclaimed on July 25, 1952.