The order arrived not through diplomatic channels but via a social media post, just hours before the annual joint military exercises between South Korea and the United States were set to begin.

US President Donald Trump, writing on his Truth Social platform, said on Monday that he was directing the Pentagon to "substantially reduce" the drills, which he described as sending an "inappropriate and hostile" signal to North Korea.

He added, pointedly, that Seoul had declined to join the US and Israel's war on Iran.

The announcement caught South Korean officials off guard, landing on a public holiday marking the country's liberation from Japanese colonial rule, and set off a scramble in Seoul and Washington.

The Pentagon said only that it was "actively working on executing" the president's directive. In Seoul, General Xavier Brunson, commander of US Forces Korea, met with Defense Minister Ahn Gyu-back at the Ministry of Defence. By Wednesday, the two militaries had settled on specifics: the exercise, already under way, would be cut from 11 days to five, with the scope of several field training drills scaled back as well.

The developments have sparked anxiety and anger in Seoul, reigniting a debate over the reliability of a security guarantee that South Korea has depended on since the 1950-53 Korean War, which ended not in peace but in an armistice.

South Korean President Lee Jae Myung has tried to project calm. Addressing his Cabinet on Tuesday, Lee said the US-South Korea alliance remained essential, but he also called for "strengthening" the country's "self-reliant defence capabilities".

On Wednesday, he struck a conciliatory tone towards Trump, calling the US president's decision "deeply infused with deliberations" to end hostility on the Korean Peninsula. In a post on X, he said he "fully empathises" with Trump's view that Seoul must shoulder its own security, and signalled a willingness to help restore freedom of navigation in the Strait of Hormuz — the energy route that Iran has blockaded — saying his government will continue talks on possible "practical contribution measures".

In parliament, Minister of Foreign Affairs Cho Hyun told anxious lawmakers that there had been no advance notice of the move. "Neither our government nor US officials knew what President Trump wrote on his Truth Social," Cho said. "We were not notified in advance."

Lawmakers on the National Assembly's defence committee were due to issue a formal response on Thursday.

On social media, reactions ranged from frustration to open anger.

"We can't trust everything Trump says," South Korean user Kim Won-tak wrote on Instagram.

Another user, Choi Min, said: "The US prioritizes practical benefits over loyalty to allies. They do it to Europe as well. We can't get rid of our dependence on the US right away, but we should work towards self-reliance."

Choi Joon-ho was blunter. Trump, he wrote, "puts his allies in a bind with tariffs, starts wars himself, and then, when they drag on, demands that they get involved… He really seems unhinged."

The Hankook Ilbo, one of South Korea's most widely read dailies, called on the country's public to confront the "stark reality of a faltering Korea-US alliance". It added that it was "absurd that the US appears to treat North Korea more respectfully than its ally South Korea".

The US president has openly criticised Seoul over its refusal to support the war on Iran.

"Wait a minute," Trump said at the Oval Office on Monday. "We have 39,000 soldiers over there, guarding you from Kim Jong Un, your next-door neighbour, and you're not going to help us on a very easy military operation in Iran? That's strange."

Opinion on the war is mixed in South Korea. Polling in late March by Hankook Research showed some 38 percent of respondents saying the government should "support the US", while 36 percent said it should remain neutral. Some 16 percent said that "we should oppose the US".

Opposition People Power Party leader Jang Dong-hyeok criticised President Lee for declining Trump's request for help with Iran, accusing him in a Facebook post of causing a "diplomatic disaster" and "dismantling the South Korea-US alliance". Jang warned that the alliance could "unravel" and that Trump "could even push for a full withdrawal" of US troops from South Korea.

In an editorial on Tuesday, the conservative JoongAng Ilbo argued that Lee's administration bore some of the responsibility, writing that an alliance "is not a one-sided favour but a reciprocal relationship". It urged the government to demonstrate its contributions, "from investment and trade commitments to support for navigation through the Strait of Hormuz".

The Korea Herald argued that an alliance "can survive disagreement" but is harder to sustain "when one side learns about a major security decision from social media". Its editorial board suggested that the gesture towards Pyongyang, arriving as US midterm elections in November approach, might have as much to do with shifting attention from "the protracted Iran war" as with security on the Korean Peninsula. The paper urged Seoul to respond "without either panic or complacency" and reduce its "vulnerabilities created by over-dependence on decisions made in Washington".

The episode has also revived a longstanding debate over whether South Korea should pursue its own nuclear weapons.

Cha Du Hyeogn, vice president of the Asan Institute for Policy Studies, said the predictability Seoul has long assumed from Washington can no longer be taken for granted. "If South Korea is not willing to play the role that the United States expects of an ally, Washington can also reconsider elements of its security commitment," Cha said. He added that unresolved trade negotiations — including a South Korean pledge to invest $350 billion in the US in return for tariff relief — may also be weighing on the relationship.

Cha also flagged the challenge ahead for Trump on North Korea. "If the US were seriously pursuing negotiations with North Korea, denuclearisation would still have to be on the table," he said. "Given that Trump went to war with Iran over its nuclear ambitions, it would be difficult for the US to pursue talks with North Korea without addressing its nuclear programme."

Kim Jae Chun, professor of international relations at Sogang University, said Trump's unilateral decision to scale back the joint exercises "without consulting the South Korean government can be perceived by South Koreans as a weakening of the US commitment to their security". He warned that if that confidence deteriorates further, "it could strengthen the argument in South Korea for self-nuclearisation".

Kim cautioned against "overreacting", however. Describing South Korea as a "junior partner" in the alliance, he said the country "may have to compromise some degree of autonomy in exchange for securing a more reliable US security commitment".

Many in Seoul, he added, still hope that a thaw between Trump and North Korea's Kim could prove to be "a first step toward a more stable peace regime on the Korean Peninsula".