UPDATED AT 4:11 P.M. EDT

South Korea on Tuesday was facing its worst confrontation of right and left political forces in decades — with the president declaring martial law and his foes vowing to defy him. The conservative president, Yoon Seok-yul, has the power to order his forces onto the streets against the leftist-led politicians who dominate the national assembly and the labor unions.

Then, as surprisingly as he declared martial law, Mr. Yoon early Wednesday, Seoul time, suddenly withdrew his decree.

“I declared emergency martial law with my resolute intent to save the nation in the face of anti-state forces that attempt to paralyze the nation’s essential function and the constitutional order of free democracy,” Seoul’s Yonhap News quoted him as saying. “But there was a demand from the National Assembly for the lifting of martial law, [I] have withdrawn troops mobilized to execute martial law affairs.”

At one point Tuesday, soldiers reportedly surrounded the assembly. They have, though, withdrawn. The showdown erupted after Mr. Yoon, who is suffering from low ratings in political polling, declared martial law, the first time a president has resorted to that since the assassination of the president, Park Chung-hee, in 1979.

The crisis caught many analysts in Korea by complete surprise. It comes at a moment of political transition in America, with President-elect Trump assembling a foreign policy team that, it is clear, is going to be sorely tested in the coming weeks and months. Mr. Yoon is trying hard to get in Trump’s good graces. He’s said he’s practicing golf in hopes of meeting the president-elect on the golf course as well as at the Oval Office. 

Mr. Yoon is not saying so, but he may have been motivated to declare martial law in anticipation of Trump’s presidency. One big concern is whether Trump will want to cut down the number of American GIs in South Korea  — now about 28,500, many of them headquartered at America’s biggest overseas base, Camp Humphreys, 40 miles south of Seoul.

Mr. Yoon, frustrated by politicians who refused to yield to his budget requests and sought to impeach senior officials —  including the man in charge of his budget and his chief prosecutor — decided, without warning, that now was the time to act. A retired American Army colonel and analyst on Korea, David Maxwell, told the Sun Mr. Yoon’s “lowest approval rating counterintuitively gives him the opportunity to act.”

Meaning that Mr. Yoon may have figured he had “nothing to lose but with the hope that it will eradicate the radical left.” A veteran of five tours in South Korea with American special forces, Mr. Maxwell noted the decision was fraught with danger — including “miscalculation and misunderstanding by security forces who could interpret protests incorrectly and inappropriately use force that could cause escalation.” 

Mr. Yoon, though, saw a far worse danger in intractable leftist-led obstructionism, which he described on Korean television as  “anti-state activities.” His order establishing martial law reflected the suspicion that Communist Chinese agents and saboteurs have infiltrated Korean society, from the biggest conglomerates to the biggest labor unions, some controlled by leftist zealots, many of whom have never worked for a factory.

Mr. Yoon, despite his low ratings in the polls, sought to appeal to this widespread view, saying martial law was “aimed at eradicating pro-North Korean forces and to protect the constitutional order of freedom.” As reported by the South’s Yonhap News, he promised  to “eliminate anti-state forces as quickly as possible and normalize the country.”

Might he, though, have overplayed his hand, which includes the power of the Korean president over the armed forces? Mr. Maxwell said there was always the danger that this could “destroy” the Republic of Korea’s conservative movement “by delegitimizing it and ensure a win by the opposition in the next election.”

“How will Kim Jong-un exploit this?” Mr. Maxwell asked.  

The opposition Minju, or Democratic Party, holding a decisive majority over Mr. Yoon’s People Power Party, at once maneuvered to pass a bill nullifying martial law, but soldiers and police blocked the entrance to the assembly.

Protesters with signs declaring “No Martial Law” surged around the gates to the park surrounding the assembly, and many more were expected to descend on central Seoul, the scene of a massive outpouring that led in 2017 to the ouster, jailing, and impeachment of the South Korean president, Park Geun-hye, daughter of Park Chung-hee.

Mr. Yoon, hoping to forestall a similar outpouring, is ordering tens of thousands of police and soldiers into the downtown area, the scene of enormous demonstrations periodically over the years. 

The country is now tensed for episodes of violence even as South Korea confronts North Korea, whose leader, Kim Jong-un, has formed an alliance with Russia’s president and is sending arms and men to support the Russians in Ukraine.

The declaration of martial law was not expected. “I was completely shocked,” a Korean woman told the Sun via phone from Seoul. “Nobody imagined anything like it.” 

Although Korean leftists and liberals may appear bent on opposing martial law, they are not as popular as some might believe. “The Chinese are trying to take over the country,” a commentator who goes by the name “Dr. Kim” said in a podcast. “Many members of the assembly are pro-North,” meaning that they are sympathetic with Mr. Kim’s shockingly dictatorial regime.