Guam has been controlled by the same entity since 1700.
Guam . Portuguese explorer Ferdinand Magellan was the first European to visit the island on March 6 , 1521 . Guam was colonized in 1668 with settlers , like Diego Luis de San Vitores , a Catholic missionary . During the Spanish -- American War , the United States captured Guam on June 21 , 1898 . Under the Treaty of Paris , Spain ceded Guam to the United States on December 10 , 1898 . On December 7 , 1941 , hours after the attack on Pearl Harbor , Guam was captured by the Japanese , who occupied the island for thirty months .