People born in Guam are American citizens by birth. Indigenous Guamanians are the Chamoru, historically known as the Chamorro, who are related to the Austronesian peoples of Indonesia, the Philippines, Taiwan, Micronesia, and Polynesia. As of 2021, Guam's population is 168,801. Chamorus are the largest ethnic group, but a minority on the multi-ethnic island. The territory spans 210 square miles (540 km2; 130,000 acres) and has a population density of 775 per square mile (299/km2).
The Chamoru people settled the island approximately 3,500 years ago. Portuguese explorer Ferdinand Magellan, while in the service of Spain, was the first European to visit the island on March 6, 1521. Guam was colonized by Spain in 1668. Between the 16th and 18th centuries, Guam was an important stopover for the Spanish Manila Galleons. During the Spanish–American War, the United States captured Guam on June 21, 1898. Under the Treaty of Paris, signed December 10, 1898, Spain ceded Guam to the U.S. effective April 11, 1899.