Lee Jae-myung will be one of the most powerful presidents in decades in South Korea, but he is taking over during a perilous political atmosphere.

If there is one characteristic that defines Lee Jae-myung, South Korea’s new president, it’s that he is a survivor.

He has survived criminal charges, a near-fatal stabbing attack and the martial law enacted by his fiercest enemy, former President Yoon Suk Yeol. Now he is taking on what may be his toughest test yet. He must lead a deeply divided nation through daunting challenges, both at home and abroad.

Mr. Lee, who won South Korea’s presidential election after his opponent conceded early Wednesday, takes office as one of the most powerful presidents that South Korea has elected in recent decades. Much of South Korea’s political power is concentrated in the presidency, and Mr. Lee will also wield considerable control over the National Assembly, where his Democratic Party holds a large majority of seats. ​

​The political turmoil set off by Mr. Yoon’s short-lived declaration of martial law and his subsequent impeachment and removal​ has exposed a country deeply fractured between the left and right, between generations and between genders. South Korea is ​facing ​mounting pressure from its sole military ally, the United States, even as the nuclear threat from North Korea grows. President Trump has not only slapped South Korea’s export-driven economy with heavy tariffs but also demanded that it pay more to keep American troops on its soil.

​Mr. Lee warned that the second Trump administration was bringing “the law of the jungle” into international relations. But Mr. Lee, 61, who rose to become a charismatic leader of South Korea’s biggest political party after working in a sweatshop in his teens, said he would also rise to this challenge, with “pragmatic diplomacy centered on the national interest.”

“I will crawl between his legs if necessary, if that’s what I have to do for my people,” Mr. Lee said this week about Mr. Trump. “But I am not a pushover, either. South Korea also has quite a few cards to play in give-and-take negotiations.”

​For Washington, Mr. Lee could represent a sharp departure from Mr. Yoon, who antagonized China and North Korea and aligned his country more firmly with the United States in the strategic competition between Washington and Beijing. Mr. Yoon also won plaudits from Washington for leaving behind his country’s historical grievances to improve ties with Japan, Korea’s former colonial master.

Mr. Lee​ and other leaders of his Democratic Party ​are not as enthusiastic about warming up to Japan, which they accused of failing to come clean on the brutalities it inflicted during its colonial rule. Mr. Lee has said he will also seek to mend ties with North Korea and China, South Korea’s largest trading partner, while maintaining a strong military alliance with the United States.

​Such a stance could set off alarms in Washington. U.S. officials have been urging South Korea to increase trilateral cooperation with the United States and Japan to contain North Korea and China. Even before Mr. Lee won the election, a warning came out from Washington.

“We know that many countries are tempted by the idea of seeking both economic cooperation with China and defense cooperation with the United States,” Defense Secretary Pete Hegseth said on Saturday. But “economic dependence on China,” he warned, “only deepens their malign influence and complicates our defense decision space during times of tension.”

Mr. Lee has spurned his conservative enemies’ accusation that he was “pro-China” and “anti-U.S.” by repeatedly stressing the importance of the alliance with Washington and trilateral cooperation with the United States and Japan for regional security.

“But that doesn’t mean that we have to make enemies of other nations,” he said.

South Korea’s political turmoil began when Mr. Yoon declared martial law in December to try to remove Mr. Lee and other “anti-state forces” in the National Assembly, who he said had repeatedly used their majority power to obstruct his policies. Mr. Yoon’s brazen move outraged South Koreans, who swarmed the streets. Mr. Yoon was impeached by the Assembly and in April was officially removed from office by the Constitutional Court, opening the way for Tuesday’s election.

During his short-lived military rule, Mr. Yoon had planned to arrest Mr. Lee, according to prosecutors. But Mr. Lee was the biggest political beneficiary of the​ dramatic collapse of Mr. Yoon​’s misjudged martial law, as​ many South Koreans saw him as the only politician with enough political resources to lead the country.

“People are so shocked by the martial law and felt such a threat to their democracy that punishing those responsible was the dominating theme in the election,” said Park Sung-min, the chief executive of Min Consulting, a Seoul-based political advisory company. “All the potential misgivings people might have about​ Lee Jae-myung’s foreign and economic ​policies and his legal trouble became secondary.”

Mr. Lee enters office with legal uncertainty hanging over his head. He has been tried on several charges, including allegations of bribery and election-law violations, and there is debate over whether proceedings should continue or be postponed until after his five-year term. His conservative critics have accused him of using ​the election as a shield against his legal trouble.​ Mr. Lee insists that the criminal charges against him are groundless and politically motivated.

​Widely differing views on Mr. Lee’s legal trouble are emblematic of the political polarization in South Korea, which has deepened in recent years. No matter who won the election, a large portion of the population would not have accepted the result. Mr. Lee vowed to ease the problem by building “national unity.”

He has said he will not seek political revenge, but the right-wing opposition has been skeptical, citing some of Mr. Lee’s own words. He has vowed to “ferret out” and “punish” those who collaborated with Mr. Yoon in his declaration of martial law.

​Nearly all of Mr. Lee’s predecessors tried to strengthen their political grip by investigating the presidents who came before them in the name of “liquidating the past wrong,” a common phrase in South Korean politics. It has created a vicious cycle of political revenge. Of the nine presidents who have held power in the country since 1980, six have faced stiff political retribution: Two were expelled from office through impeachment, and four were convicted of criminal charges and imprisoned after they left office. Another former president killed himself while he was under criminal investigation.

“It will be a tough balancing work for Lee Jae-myung: On one hand, he must punish those involved in insurrection during the martial law, and, on the other hand, he must avoid looking like he is indulging in political revenge,” said Ahn Byong-jin, a professor of political science at Kyung Hee University in Seoul. “He probably is taking office in a more difficult time than any other South Korean leader ​has faced in recent years.”

Eventually, Mr. Lee will be ​judged by his performance in reviving South Korea’s economy​, analysts said. South Koreans cited ​fixing the economic slump as the most urgent task for the new leader.

“The election was about punishing ​the martial law,” said Shin Yul, a political scientist at Myongji University in Seoul. “But what the people most want​ is stability and a better economy.”