US has 'grave concern' as South Korea imposes, then overturns, martial law

After being caught off guard by South Korean President Yoon Suk Yeol's decision to declare martial law on Tuesday, the Biden administration declined to condemn the move as it struggled to explain the close U.S. ally's rare but dramatic episode of political turmoil.

Officials from the White House, Pentagon and State Department all said they were in close contact with their South Korean counterparts on Tuesday as they tried to assess the situation. President Joe Biden, who is currently traveling in Angola, also said he was "just getting briefed" on developments after wrapping up an address.

Earlier on Tuesday, Deputy Secretary of State Kurt Campbell said the administration was watching developments unfold in South Korea with "grave concern." But Campbell and other officials across Washington also underscored what they said was an "ironclad" partnership between the United States and South Korea.

Despite the close alliance, both the White House and State Department said the U.S. was not given advance notice of Yoon's intention to impose martial law. U.S. officials did not publicly denounce Yoon's decision but also did not endorse his allegations that South Korea's opposition party was involved in "anti-state" activities with North Korea -- Yoon's stated reason for temporarily suspending civilian rule.

"This is an incredibly fluid situation. I'm not going to jump to any conclusions at this point," State Department deputy spokesman Vedant Patel said on Tuesday. "Every hope and expectation is that any political disputes will be resolved peacefully and in accordance with the rule of law."

That resolution appeared to be coming into focus just hours after Yoon's declaration. In a dramatic scene, South Korean lawmakers worked their way through throngs of protesters and military vehicles to gather in the National Assembly building during the middle of the night, where members of the body who were present unanimously voted to end martial rule.

Soon after, Yoon complied with the decision by lifting his martial law declaration and said troops deployed to enforce the order had already been withdrawn.

"We are relieved President Yoon has reversed course on his concerning declaration of martial law and respected the ROK National Assembly's vote to end it. Democracy is at the foundation of the U.S.-ROK alliance, and we will continue to monitor the situation," a U.S. National Security Council spokesperson told ABC News in a statement.

"This is a bullet dodged," said retired U.S. Navy Rear Adm. Mark Montgomery. "I think the U.S. government probably was hoping for what happened and stayed muted to see if it would happen."

Montgomery, now a senior director at the Foundation for Defense of Democracies, said the apparent outcome in South Korea -- democratic processes triumphing over an authoritarian instinct -- was far from guaranteed.

"I consider South Korea a beleaguered democracy. There's constant pressure from North Korea, constant pressure from China, and now you have Russia giving things to North Korea," he said. "They are, like Taiwan, like Ukraine, like Israel, a beleaguered democracy on the front lines against this access."

Still, the tumult poises fresh frustration for the Biden administration and its legacy of promoting democracy abroad, as well as its strategy of containing China by bolstering other partnerships in the region.

South Korea has been a favored ally through Biden's term in office. The president honored Yoon with a state visit in April and tapped the country to host the Summit for Democracy -- an annual convention created by Biden with the intent of promoting democratic values.

The U.S. military also has a large footprint in South Korea, with nearly 30,000 troops stationed in the country. In its first security alert since Yoon moved to impose martial law, the U.S. Embassy in Seoul warned Americans in the country that "the situation remains fluid" even though Yoon has announced that the order has been lifted.

The U.S. has also been committed to helping South Korea defend itself since the two countries signed a mutual defense treaty at the end of the Korean War in 1953.

Even as South Korea looks to turn the page from its brush with martial law, Montgomery said its alliance with the U.S. is still likely to face some uncertainty as Yoon confronts mounting backlash.

"Yoon walked across the Rubicon," he said. "His political life is in question."

ABC News' Molly Nagle contributed to this report.