For decades, South Korea has been one of the most important U.S. allies in Asia — not only because nearly 30,000 American troops are stationed there, but also because it stands as a beacon of democracy in a region where powerful authoritarian nations vie with democratic ones.

President Biden has put a special emphasis on South Korea, choosing it as the first non-U.S. site for his annual international conclave, the Summit for Democracy. And in 2023, he hosted President Yoon Suk Yeol for a state dinner at the White House, where the tuxedo-clad Mr. Yoon sang “American Pie” to an adoring audience. Mr. Biden has also relied on Mr. Yoon to provide munitions for Ukraine’s defense against the Russian invasion.

Now, with Mr. Yoon having imposed martial law after wildly accusing the opposition party of conspiring with North Korea to undermine him, the American relationship with South Korea could face its biggest test in decades.

And Mr. Biden, who has used democracy versus autocracy as a defining framework of his foreign policy, will have to make hard choices on how to handle the crisis, after years of cultivating relations with Mr. Yoon, a conservative leader, and enhancing military ties to better counter China, North Korea and Russia.

Mr. Yoon’s move appeared to catch the Biden administration by surprise. Hosting Mr. Yoon at the White House in April 2023, Mr. Biden told Mr. Yoon that the two men “both understand that our democracies and our people are our greatest sources of strength.”

On Tuesday afternoon in Washington, the White House National Security Council released a terse statement, using an abbreviation for South Korea’s formal name, the Republic of Korea: “The administration is in contact with the R.O.K. government and is monitoring the situation closely as we work to learn more. The U.S. was not notified in advance of this announcement. We are seriously concerned by the developments we are seeing on the ground in the R.O.K.”

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