Moon Jae-in, South Korea’s new president, faces U.S. diplomatic test

As a candidate for president, Moon Jae-in sought to end a decade of conservative dominance of South Korean politics by telling voters he would be a leader who could “say no to America.”

Now, the 64-year-old onetime human rights lawyer will have to find a way to get to “yes” with President Trump while dealing with an increasingly assertive North Korea across the border, rising tensions in the region and a sputtering economy and rising corruption at home that have called into question the South Korean way of doing business.

Although there is no transition period, Mr. Moon is likely to enjoy at least a brief honeymoon by putting a decisive end to a turbulent half-year of politics in Seoul, capped by the impeachment and removal of President Park Geun-hye on corruption charges. The new president will be assembling his government and trying to build a majority in the parliament even as Ms. Park’s corruption trial moves ahead.

Mr. Moon is expected to revive the “Sunshine Policy” of outreach and engagement with the regime of North Korean leader Kim Jong-un, and has questioned the installation of a U.S. missile defense system aimed at frustrating the North’s nuclear and conventional arsenal. But the Trump White House said it hoped the election result would “strengthen the alliance” between Washington and Seoul.

White House press secretary Sean Spicer said the U.S. joined with its ally in “celebrating their peaceful, democratic transfer of power.”

After a campaign that focused on domestic issues, Mr. Moon took 41 percent of the vote, easily besting conservative Hong Joon-pyo and centrist Ahn Cheol-soo, who received 24 percent and 21 percent, respectively.

The low-key, bespectacled president-elect smiled and waved to the crowds as supporters filled Gwanghwamun Square in the heart of Seoul, The Associated Press reported. Because Ms. Park was already removed from office, Mr. Moon will officially become president as soon as election officials confirm the results Wednesday, eschewing the usual two-month transition period.

“It’s a great victory by a great people,” Mr. Moon told supporters. “I’ll gather all of my energy to build a new nation.”

Despite the clear result, Mr. Moon and his Democratic Party have work to do, holding just 40 percent of the single-chamber, 299-seat assembly. The president will have to forge alliances to secure a working legislative majority. Voter turnout was 77.2 percent, the highest in 20 years, despite occasional rainy skies.

Despite the White House statement, there are fears in both capitals that the U.S. and South Korea may be on a collision course as Mr. Trump tries to rally countries in the region to put more pressure on North Korea and isolate it for its growing nuclear arsenal and series of ballistic missile tests. Mr. Moon also has promised a close scrutiny of the Pentagon’s just-installed THAAD anti-missile system, which has angered China, the South’s biggest trading partner, although the new president stopped short of saying he would demand THAAD’s removal.

The Trump challenge

Many of Mr. Moon’s supporters, including a large percentage of younger South Koreans, see the more aggressive American stance — as well as Mr. Trump’s criticism of the free trade deal with South Korea signed under President Obama — as an equal threat to stability on the peninsula, said Duyeon Kim, a visiting fellow at the Korean Peninsula Future Forum.

“South Koreans are more concerned that Trump, rather than [Mr. Kim], will make a rash military move, because of his outrageous tweets, threats of force and unpredictability,” she wrote recently. “It is crucial that Trump and the next South Korean president strike up instant, positive chemistry in their first meeting to help work through any bilateral differences and together deal with the North Korean challenge.”

There has reportedly been friction between Mr. Trump and his national security adviser, H.R. McMaster, over South Korea’s missile defense system. Mr. Trump said South Korea should pay for operating the system, while Mr. McMaster later called his counterpart in South Korea to assure him that the U.S. would not insist on Seoul paying for it.

A report by Bloomberg this week quoted unidentified White House advisers as saying Mr. Trump has complained that Mr. McMaster had been undermining his policies.

White House press secretary Sean Spicer said Tuesday that Mr. Trump’s relationship with Mr. McMaster is excellent and that he “values his counsel.”

But Mr. Trump has also raised the possibility of a face-to-face meeting with Mr. Kim if North Korea gives up its nuclear arsenal, and analysts say Mr. Moon will likely have to temper his outreach to Pyongyang so as not to anger the administration in Washington or other countries in the region.

Mr. Trump’s pressure strategy “cannot succeed if the South Korean president puts the brakes on this campaign,” said Dan Blumenthal, director of Asian studies at the American Enterprise Institute. “Given the sobering nature of Kim’s danger, I think Moon will continue with the tougher approach” of Ms. Park.

Mr. Moon, who lost a close race to Ms. Park for president in 2012, will have no shortage of tough issues filling his inbox as he assumes power. In addition to the standoff with the North, Mr. Moon has promised to take on the powerful industrial conglomerates known as “chaebols” that have fueled the South’s meteoric rise to prosperity, but which many South Koreans now blame as the source of corruption that helped bring down Ms. Park.

With economic growth slowing to below 3 percent and youth unemployment at over 10 percent, Mr. Moon has promised a major stimulus package of government spending to spur growth. One business project that may see new life: the Kaesong Industrial Zone, a joint North-South venture that was “temporarily” closed by the Park government in February 2016 in protest of Pyongyang’s belligerent moves.

Mr. Moon’s background also argues for a softer line on policy toward the North. He was chief of staff and key aide to President Roh Moo-hyun, the last liberal president who also pursued the Sunshine Policy. He was a sharp critic of Ms. Park’s ethical problems and has called for reforms to cut the ties between politicians and big business and to trim the powers of the president.

Ms. Park’s trial this month on bribery, extortion and other corruption charges could send her to jail for life if she is convicted, the AP reported. Dozens of high-profile figures, including Ms. Park’s longtime confidante, Choi Soon-sil, and Samsung’s de facto leader, Lee Jae-yong, have been indicted along with the ousted president.

• Dave Boyer contributed to this article, which is based in part on wire service reports.