Geopolitics › Alliances & institutions › US–South Korea alliance
US Forces Korea (USFK)
About 28,500 troops· Alliance or organization
FactTreaties, membership, measured data, events.
The roughly 28,500 American troops based in South Korea, the backbone of the alliance's deterrent against North Korea.
See it on the Geopolitics map →The picture
Formed as a command in 1957, USFK centers on the Eighth Army and the Seventh Air Force. Most troops are now in Pyeongtaek, at Camp Humphreys, often called the largest US overseas base, and Osan Air Base, home to fighter jets, after a move south from Seoul's Yongsan garrison largely completed in 2018. Its four-star commander also leads the UN Command and the Combined Forces Command. A combat brigade rotates in from the US. Congress has repeatedly discouraged cuts below 28,500.
Why it matters
The troops act as a 'tripwire': an attack on the South would hit Americans, making US involvement near-certain. That is the deterrent, and Washington's leverage in cost talks.
Key facts and numbers
Pentagon and USFK posture statements to Congress, annual defense authorization acts, and Korea's cost-sharing agreements; numbers shift with rotations, so 28,500 is the official benchmark.
What to watch
Whether Washington trims USFK or repurposes it for China or Taiwan contingencies, as 2025 press reports said the Pentagon weighed, and how Seoul would respond.
Connections (2)
▸US Forces Korea (USFK) helps uphold the armistice in Korean peninsula standoff (armistice since 1953)Widely agreed
The relationship
The armistice is overseen by the UN Command, which the USFK commander also leads, while US and South Korean forces deter a new invasion. The UN Command runs the Joint Security Area at Panmunjom and oversees armistice rules in the DMZ. Most analysts credit the US presence with keeping the war frozen for over 70 years.
Since when and why
The DMZ and Panmunjom; armistice signed July 27, 1953; USFK formed in 1957.
How solid?
Widely agreed but untestable, since deterrence is credited for what did not happen; UN Command records and North Korea's own demands for US withdrawal show the troops matter.
What to watch
North Korea calls US forces the threat; some argue South Korea could now defend itself. Whether a peace treaty would require US troops to leave is disputed.
▸US–South Korea alliance is backed on the ground by US Forces Korea (USFK)Fact
The relationship
Article IV of the 1953 treaty grants the US the right to station land, air and sea forces in and around South Korea. USFK is the alliance's physical core: its commander leads the Combined Forces Command that would fight a war, and Seoul shares its costs under multi-year agreements.
Since when and why
Mutual Defense Treaty Article IV, 1953; USFK formed in 1957; Combined Forces Command since 1978.
How we know
The treaty text, Pentagon posture statements, and the Special Measures Agreements, which set Seoul's payments.
What to watch
Whether troop numbers or roles change under Trump's pressure, and how command arrangements shift once Seoul takes wartime control.