There are school bus routes, baseball diamonds and American football fields. Soldiers queue for lunch at Taco Bell, Pizza Hut and Arby’s outlets. A postbox stamped with the logo of the US postal service stands outside the commissary stocked with American groceries. The signage is all in English and the US dollar is the currency in use.

Beyond the perimeter fence, military helicopters rise above the airfield and cut across the blue sky.

The scene is a slice of contemporary America, despite being more than 5,000km from the US mainland.

Camp Humphreys, located in the South Korean city of Pyeongtaek, is the largest American military base outside the US: 1,372 hectares (3,390 acres), nearly a thousand buildings and with a population of roughly 41,000, including American service members, their families and Korean nationals.

It serves as headquarters for United States Forces Korea (USFK), the clearest physical expression of the alliance between Washington and Seoul that has underpinned stability on the Korean peninsula since the Korean war ended in an armistice in 1953.

Yet the alliance that planted this piece of America in South Korea is now being tested. From trade tensions to security guarantees, relations under President Donald Trump are increasingly transactional in a way that has unsettled Seoul, which has long depended on Washington as a guarantor in its defence against North Korea.

“Reliability and credibility issues are worse than they were before,” says Mason Richey, a professor of international politics at Hankuk University of Foreign Studies in Seoul. The alliance retains deep operational ties, he says, but the political surface has become far more fraught.

When Trump announced earlier this month that he would withdraw 5,000 troops from Germany after the country’s chancellor, Friedrich Merz, said Washington was being “humiliated” by Iran – with the US president also threatening reductions elsewhere in Europe – Korean media raised the question of whether South Korea would be next, reviving speculation that has persisted throughout Trump’s presidency.

The defence ministry and presidential office moved quickly to deny that any troop reduction discussions were under way. Asked about reported adjustments to troop numbers and assets, USFK said the current 28,500 troops was “a baseline, not a limit or a ceiling”, and that the command’s focus was on capabilities rather than fixed numbers.

But under Trump, tensions in the alliance – including an immigration raid at a Hyundai-LG battery plant in Georgia last year, and threats to hike tariffs on South Korean goods to 25% – have begun to spill out into national security.

The US reportedly partly restricted intelligence sharing after a South Korean minister publicly identified a suspected North Korean nuclear site. And in April, fallout from a US-incorporated company’s data breach stalled talks on the development of nuclear-powered submarines.

Amid these tensions, South Korea is debating how dependent it can remain on American protection.

An advantage ‘no other US ally can replicate’

Behind the suburban veneer of Camp Humphreys is a military installation training for war. At the state-of-the-art Vandal Training Center, US and Korean soldiers run water-survival drills in a massive pool designed to simulate a helicopter crashing into the sea.

In a darkened medical room filled with artificial smoke and the piped-in sounds of combat, troops wearing night-vision goggles practise battlefield evacuations on $400,000 mannequins with severed limbs that are rigged to bleed on command. Upstairs, virtual-reality simulators allow units to run combat scenarios in almost any country or terrain in the world.

An official at the base says the readiness standard is “fight tonight”.

For years, their focus has been across the northern border. North Korea possesses nuclear weapons and ballistic missiles theoretically capable of reaching the US mainland. In late 2024 it deployed more than 12,000 troops to support Russian forces in Ukraine and is widely believed to have received advanced military technology and training from Moscow in return.

But Washington is becoming more explicit about recalibrating the division of labour. The Pentagon’s national defence strategy – published in January – states that South Korea is capable of taking “primary responsibility” for deterring North Korea, with increasingly limited US support.

Washington is also pushing to expand its mission beyond the Korean peninsula.

From Camp Humphreys, the US has China in view. The base is positioned roughly 800km (500 miles) from Shanghai and under 1,400km from Taiwan.

“Korea sits at the centre of the regional security geometry, with a positional advantage no other US ally can replicate,” a USFK official told the Guardian.

Xavier Brunson, the commander of USFK, says the base’s presence “complicates every calculation” an adversary makes.

In Seoul, though, there are fears that hosting a launchpad for US regional operations beyond North Korea could drag South Korea into unwanted conflict with China.

“Many South Koreans, particularly among more progressive constituencies, are reluctant to see USFK reoriented toward containing China,” says Jaechun Kim, a professor of international relations at Sogang University in Seoul.

“There is strong concern about entrapment in broader US-China strategic competition, particularly over potential Taiwan contingencies.”

Trump’s recent calls for allies, including South Korea, to join US-led operations in the strait of Hormuz sharpened concerns in Seoul about how far the alliance could extend beyond the peninsula.

At Camp Humphreys, four new barracks are nearing completion, and a new elementary school is under construction. The base is going nowhere, and for now at least, large-scale troop withdrawals remain unlikely. A sculpture commemorating the US-South Korea alliance stands outside the USFK headquarters.

Inscribed on one side, in Korean, are the words: “함께 갑시다” (hamkke gapsida) – we go together.