South Korea is getting a rude awakening about what happens when U.S. priorities shift: Even long-standing alliances can start to look like relationships of convenience.

Any erosion of American reliability in the Indo-Pacific weakens confidence in Washington. It also strengthens China’s narrative that the U.S. is unwilling to stay the course when its interests move elsewhere.

The reported transfer of U.S. air defense assets — including parts of the Terminal High Altitude Area Defense (THAAD) system — from the Korean peninsula to the Middle East to support operations in the Iran war is causing all kinds of alarm. The U.S. and South Korea agreed to install it in 2016, to help Seoul combat North Korea’s growing nuclear and missile threat.

President Donald Trump has also asked countries like Japan, South Korea and China, along with NATO and other allies, to send warships to the Strait of Hormuz to help commercial vessels sail through it safely. He’s threatened to delay his summit with President Xi Jinping if Beijing doesn’t agree to his demands.

Seoul hasn’t made a decision on whether to help Trump, and is wary of getting dragged into an ever deepening crisis. President Lee Jae Myung struck a careful note when he addressed the THAAD issue in parliament last week, acknowledging that the government opposed the redeployment, but conceding that nothing could be done to prevent it.

This is a difficult moment for South Koreans. It’s a reminder that the guarantees offered by their most important ally can’t be trusted. The country has already been bruised by the White House’s tariffs, and now this. To contain the damage, Trump and his team need to offer Seoul reassurance — not chaotic messaging and difficult requests.

Pyongyang has already seized the moment. It fired more than 10 ballistic missiles toward the waters off its eastern coast on Saturday, days after testing cruise missiles from a new warship. This is worrying for Lee, who said that even if the U.S. military moves air defense assets out of South Korea, it won’t seriously affect Seoul’s ability to defend itself against its nuclear-armed neighbor.

North Korea’s supreme leader Kim Jong Un is in his strongest position in years, as I have written recently. The conflict in Iran will only reinforce his perception that Washington’s attention is divided.

None of this means the U.S.-South Korea alliance is collapsing. Washington stations roughly 28,500 troops there under a mutual defense treaty codified in 1953. For decades, that alliance has anchored deterrence on a peninsula remains technically at war.

Seoul has been a willing partner, boosting defense spending in response to Trump’s calls for greater burden-sharing. It has also pushed through a special bill to invest $350 billion in the U.S., in an attempt to placate the president over his arbitrary tariff program.

But this plays neatly into China’s narrative. It objected vehemently to the installation of THAAD around a decade ago, notes Edward Howell, a lecturer in international relations at the University of Oxford and author of the upcoming book, A New Axis of Upheaval: North Korea, Russia, China, Iran. Chinese officials have long argued the U.S.-made system could be used to track its own missiles in a Taiwan conflict, weakening Beijing’s nuclear deterrence, he told me. If parts of it are now redeployed elsewhere, “it will raise broader questions about how reliable U.S. alliances are in the Indo-Pacific,” Howell said.

Seoul spent years dealing with economic and political fallout from China for hosting THAAD. The costs ran into the billions, as relations with its largest trading partner sunk to historic lows. But it absorbed that backlash and argued the system was essential to counter North Korea’s evolving missile threat. Washington has left Seoul wondering whether those sacrifices were made in vain.

This harsh lesson is being absorbed in real time by South Koreans, who may start pushing their government to rely less on the U.S. for protection, and strengthen their own defenses by expanding domestic weapons programs and even considering the idea of a homegrown nuclear arsenal. Public opinion polls have consistently shown widespread support for developing an independent deterrent. If doubts about American reliability deepen, that pressure will grow.

This could trigger a regional arms race, undermining the non-proliferation framework America as spent decades trying to uphold. Lee has also been making overtures to Beijing, which could open up more space for China to expand its influence.

How the U.S. balances its priorities will reinforce longer-term concerns about its security guarantees. Japan, which hosts the largest concentration of forces in Asia, will be particularly sensitive to any signs that these commitments are becoming conditional. Its defense minister confirmed Monday there were no plans to send warships to the Strait of Hormuz.

In the absence of a clear response, Washington could simply clarify whether the reported THAAD redeployment is temporary. Outlining other measures to protect Seoul and preserve deterrence on the peninsula would go a long way to calm nerves. Providing details about additional surveillance, rotational deployments and expanded exercises might also help to reassure anxious citizens. A joint U.S.-South Korea statement should outline that treaty obligations remain iron-clad.

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This column reflects the personal views of the author and does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

Karishma Vaswani is a Bloomberg Opinion columnist covering Asia politics with a special focus on China. Previously, she was the BBC's lead Asia presenter and worked for the BBC across Asia and South Asia for two decades.

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