After nine years of conservative rule, a progressive will now occupy the Blue House. Moon Jae-in, the Democratic Party candidate, handily won Tuesday’s presidential election in South Korea. His victory will likely result in serious and significant swings in South Korean policy in ways that affect Japan and other nations in the region. There is no time to lose in preparing for those shifts and to build relations with the new government to minimize their impact on Japan.

Moon is a former human rights lawyer and progressive politician who served as chief of staff to the last liberal president, Roh Moo-hyun. He was one of Roh’s closest friends; he was in charge of Roh’s funeral after he committed suicide and helped settle his affairs. That experience no doubt will have an impact on his presidency and his management of Korean affairs.

Moon won a convincing victory in Tuesday’s ballot. He claimed 41 percent of the vote, cleanly besting conservative candidate Hong Joon-pyo with 24 percent of the vote, and the centrist software magnate Ahn Cheol-soo, who claimed 21.4 percent of ballots. With turnout at 77 percent, the highest level in two decades, Moon can claim a genuine mandate.

Moon rides a tide of popular disgust with conservative politics. He was narrowly defeated by Park Geun-hye in the 2012 presidential election, an outcome that deepened the bitter divisions that characterize South Korea’s political world. But Park’s impeachment last year, and its unanimous confirmation by the Constitutional Court in March, created a surge of support for Moon, who has long enjoyed an image as a clean politician. Opinion polls taken just before the election showed that the top concern for the country’s voters was “deep-rooted corruption” and a desire to promote reform; second on that list was economic revival.

If Moon is to succeed in those tasks, he must tackle the chaebol, the huge industrial conglomerates that dominate the South Korean economy and have outsized influence in its politics. The left in South Korea has long sought to reduce their power and size, but have invariably failed. The Park scandal also ensnared some of the country’s largest chaebol and Moon will try to use those entanglements to press long-sought reform.

South Korea’s economic rejuvenation is of importance to Japan, but of more immediate significance, and concern, are potential swings in Seoul’s foreign policy in the wake of Moon’s win. Prime Minister Shinzo Abe, in commenting that he “looks forward to working with” the new president, said the two countries are “the most important neighbors that share strategic interests.” A good bilateral relationship is essential for peace and stability in the Asia-Pacific region. It is worrisome then that Moon has said that he wants to renegotiate the “comfort women” agreement that Tokyo and Seoul reached in December 2015 and helped the two countries move beyond that contentious issue. To his credit, Moon said earlier this month that he will not use that issue as the “starting point” for relations with Japan, but the temptation to play that card will be strong. Japan should not give South Korea a reasons to do so.

Tensions will also arise from Moon’s desire to reclaim South Korea’s leading role in relations with North Korea. Moon backs the Sunshine Policy of his progressive predecessors, Presidents Kim Dae-jung and Roh Moo-hyun, which sought to engage Pyongyang as a way of reducing tensions. That policy put less emphasis on the North’s denuclearization and instead offered significant financial resources to Pyongyang to convince it of Seoul’s good intentions. But the policy was undercut by the North’s cheating on commitments under the 1994 Framework Agreement that capped its nuclear ambitions and reneging on promises made during the six-party talks also involving Japan, the United States, China and Russia.

Pyongyang would have nothing to do with conservative presidents in Seoul who flatly acknowledged — and decried — the North Korean threat, hoping to mobilize the left in South Korea to take up its cause. Moon has said that he favors a “bold blueprint” to deal with North Korea. He would resume economic ties to encourage reform in the North, and would likely push for the reopening of the Kaesong Industrial Complex. In return, he looks ready to accept a freeze on nuclear tests.

The North Korean leadership hopes Moon will break with Tokyo and Washington and join China in pushing for the U.S. and others to engage without preconditions. The prospect of a rupture between the U.S., Japan and South Korea is now real, and nothing would delight North Korea more.

Moon must thread the needle. He wants to lead in relations with Pyongyang, but he needs partners to get the North’s attention. China has connections to Pyongyang and economic influence, but Beijing’s price for help is that Seoul make more distance between itself and the U.S. Yet the U.S. provides the best guarantee of South Korean sovereignty in the face of both Chinese and North Korean threats. Japan has already indicated its readiness to reach out to the new president and help him make the right choice.

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