Why is the US Navy at Vieques, Puerto Rico? Puerto Rico: From Spanish Colony to American Military Bastion After 400 years of Spanish colonial rule, Puerto Rico became a possession of the United States as a direct result of the Spanish-American War of 1898. Right after the invasion the US established a military government, which lasted up to 1900; thereafter, the Foraker Act of 1900 authorized the President of the United States to appoint a civilian governor. In 1917 the Jones Act granted US citizenship to all island residents. In 1948 Puerto ....