Guam is the largest and southernmost of the Mariana Islands, and the largest island in Micronesia. Guam is one of the 17 Non-Self-Governing Territories under supervision of the Special Committee on Decolonization of the United Nations, whose goal is to bring the colonization to an end.oday, Guam's economy is supported by its principal industry, tourism, which is composed primarily of visitors from Japan. Guam's second largest source of income is the United States Armed Forces.