Claim: Guam's first Catholic church was founded by Diego Luis de San Vitores.

Guam -LRB- -LSB- ˈɡwɑːm -RSB- or -LSB- ˈɡwɒm -RSB- Chamorro : Guåhån -LSB- ˈɡʷɑhɑn -RSB- formally the Territory of Guam -RRB- is an unincorporated and organized territory of the United States . Located in the western Pacific Ocean , Guam is one of five American territories with an established civilian government . The capital city is Hagåtña and the most populous city is Dededo . In 2015 , 161,785 people resided on Guam . Guamanians are American citizens by birth . Guam has an area of 210 sq mi -LRB- 544 km ² -RRB- and a population density of 770/sq mi -LRB- 297/km ² -RRB- . It is the largest and southernmost of the Mariana Islands and the largest island in Micronesia . Among its municipalities , Mongmong-Toto-Maite has the highest density at 3,691 / sq mi -LRB- 1,425 / km ² -RRB- , whereas Inarajan and Umatac have the lowest density at 119/sq mi -LRB- 47/km ² -RRB- . The highest point is Mount Lamlam at 406 m above sea level .   The Chamorros , Guam 's indigenous people , settled the island approximately 4,000 years ago . 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 . Between the 16th century and the 18th century , 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 , Spain ceded Guam to the United States on December 10 , 1898 . Guam is among the seventeen Non-Self-Governing Territories of the United Nations .   Before World War II , Guam and four other territories -- American Samoa , Hawaii , Wake Island , and the Philippines -- were the only American jurisdictions in the Pacific Ocean . On December 7 , 1941 , hours after the attack on Pearl Harbor , Guam was captured by the Japanese , who occupied the island for thirty months . During the occupation , Guamanians were subjected to beheadings , forced labor , rape , and torture . Guam endured hostilities when American forces recaptured the island on July 21 , 1944 ; Liberation Day commemorates the victory . Since the 1960s , the economy has been supported by two industries : tourism and the United States Armed Forces .