From the Overview: "Though geographically only about three times the size of Washington, DC, and with a population of about 5.9 million, the city-state of Singapore exerts economic and diplomatic influence on par with much larger countries. Its stable government, strong economic performance, educated citizenry, and strategic position along key shipping lanes afford it a large role in regional and global affairs. For the United States, Singapore has been a partner in both trade and security initiatives and an advocate of a strong U.S. role in the Asia-Pacific region. At the same time, Singapore's leaders have aimed to maintain close relations with China, and to maintain positive ties with all regional powers. [...] Although not a U.S. treaty ally, Singapore is one of the strongest U.S. security partners in the region. A formal strategic partnership agreement allows the United States to access Singaporean military facilities and promotes cooperation on issues ranging from counterterrorism to counter-proliferation."
CRS In Focus, IF10228
Congressional Research Service: https://crsreports.congress.gov/