President Yoon Suk Yeol shakes hands with U.S. President Joe Biden during a meeting at the Lima Convention Center in Lima, Peru, Nov. 15 (local time), on the sidelines of the Asia-Pacific Economic Cooperation summit. Courtesy of presidential office

President Yoon Suk Yeol's abrupt declaration of emergency martial law — swiftly overturned by the National Assembly early Wednesday — appears to have put South Korea's decades-old alliance with the United States to the test.

Officials in Washington were seemingly caught off guard by Yoon's martial law declaration, which proceeded without prior notice. More broadly, the political turmoil surrounding the martial law saga could lead to concerns about the Yoon government's commitment to democratic principles, which have served as the cornerstone of the Seoul-Washington alliance.

On Tuesday night, the president announced the declaration of martial law in a surprise live briefing, stating that the measure was intended to "eradicate pro-North Korean forces and protect the constitutional order of freedom."

However, the move faced immediate backlash from the National Assembly, which unanimously voted against the martial law decree. Hours after the vote, Yoon's Cabinet approved a motion to lift the martial law.

U.S. Secretary of State Antony Blinken welcomed the swift reversal, Tuesday.

"The United States has watched closely developments over the last 24 hours in the Republic of Korea (ROK). We welcome President Yoon's statement that he would rescind the order declaring emergency martial law in accordance with the ROK Constitution, after the National Assembly's unanimous vote to reject the declaration," Blinken said in a statement.

"We continue to expect political disagreements to be resolved peacefully and in accordance with the rule of law. We reaffirm our support for the people of Korea and the U.S.-ROK alliance based on shared principles of democracy and the rule of law," he added.

Soldiers prepare to enter the main hall of the National Assembly in Seoul early Wednesday, following President Yoon Suk Yeol's declaration of martial law the previous night. Yonhap

Before the law was lifted, a White House National Security Council (NSC) spokesperson said it was monitoring the situation with "serious concern," noting that the U.S. authorities had not been notified in advance of Yoon's announcement.

U.S. President Joe Biden, who was traveling in Angola at the time, was briefed on the matter shortly after the announcement.

In a breifing held in the following hours, Pentagon press secretary Maj. Gen. Pat Ryder said there was no change in the military readiness level of United States Forces Korea (USFK) in the wake of the martial law declaration, saying, "We're closely monitoring the situation, but I'm not aware of any force posture changes."

When contacted by the The Korea Times, Wednesday, a USFK spokesperson declined to comment whether it had been notified in advance of Yoon's announcement. But the official noted that the U.S. troops and their families have been advised to refrain from non-essential travel in the wake of the martial law.

"We encouraged everyone to avoid non-essential travel. We urge everyone to avoid large crowds, any possible protests, demonstrations, and avoid (South) Korean government offices in the Seoul metropolitan area," the spokesperson said.

Analysts suggest that the U.S. government appears deeply wary, if not shocked, by Yoon's martial law bid — the first such move by a South Korean leader since 1979.

"Blinken's statement stressing 'shared principles of democracy and the rule of law' signals that the U.S. government is closely watching with concern how the situation could ultimately undermine democratic values in the wake of the martial law," a local defense analyst said, speaking on condition of anonymity.

He also noted that the recent developments in Seoul could create an uncomfortable situation for the Biden administration, which has long regarded South Korea as a "champion of democracy."

Since Yoon's inauguration, Seoul and Washington have reached significant milestones in their alliance, prioritizing security and economic cooperation buttressed by shared democratic values.

"Given the current state of affairs in the U.S. — Biden's lame-duck status and the administration's transition period — our government may have felt it less necessary to share the details," the analyst added.

A TV screen at Seoul Station shows news report, Wednesday, about President Yoon Suk Yeol's declaration of martial law, Tuesday. Korea Times photo by Choi Won-suk

Following South Korea's short-lived declaration of emergency martial law, a meeting of the Nuclear Consultative Group (NCG) with U.S. officials that was set to take place on Wednedsay in Washington has been indefinitely postponed, according to the South's Ministry of National Defense.

The NCG is a bilateral security framework that aims to strengthen the credibility of U.S. extended deterrence.

But Cha Du-hyeogn, a senior researcher at the Asan Institute for Policy Studies, said the martial law fiasco itself is unlikely to have an immediate impact on relations with the U.S.

"Sharing specifics on domestic issues with allies is not always necessary," Cha said. "However, the instability in domestic policies stemming from the martial law issue could affect relations with allies over the long term. Serious domestic challenges may lower leverage in diplomacy, as allies want consistency and predictability in foreign relations."

Japan is also closely monitoring the developments in South Korea, as the ongoing situation may affect Prime Minister Shigeru Ishiba's potential visit to Seoul next month.

"We have been monitoring developments since the overnight declaration of martial law with particular and grave concerns," Ishiba was quoted as saying by Kyodo News. "We will do our utmost to ensure the safety of Japanese people in South Korea."

He added that "nothing specific has been determined" about his potential trip to Korea.

If realized, it would mark Ishiba's first visit to Seoul since he took office in October.