Congratulations are in order to Yoon Suk-yeol, the pro-American conservative who just won the presidency of Free Korea. The margin of victory might have been slim, but the stakes are enormous. It’s a turn from the line of appeasement that the outgoing president, Moon Jae-in, has been pursuing in respect of communist North Korea. We can’t help but think that Korea’s voters had a weather eye on the war in Ukraine.

We’ve been worried about South Korea for some time, marked in a spate of editorials in the spring of 2017. That was when what we think of as a coup-by-criminal-prosecution ousted the 11th president of free Korea, Park Geun-hye, the hard-line daughter of the republic’s third president, Park Chung Hee, and threw her in the clink on charges of corruption and abusing her powers. To us it was a shocking turn for Korea’s democracy.

Incredibly, the man who prosecuted Ms. Park was Mr. Yoon, then chief prosecutor under Mr. Moon. Mr. Yoon also pursued Ms. Park’s predecessor, Lee Myung-bak, a former Hyundai chairman, whose corrupt dealings were more serious than the charges against Ms. Park but still dubious, in our view. Mr. Moon has pardoned Ms. Park while Mr. Lee remains in prison. It will be up to Mr. Moon to free him.

Mr. Yoon’s ability to function effectively will depend in no small measure on his success in dealing with issues of corruption involving both conservatives and leftists.  On geopolitics, though, the outgoing president, Mr. Moon,  shares much of the outlook toward North Korea as did Ms. Park and Mr. Lee, both of whom called for aid to North Korea with one proviso. North Korea would have to give up the A-bomb.

Mr. Yoon is likely to offer much similar terms — vast quantities of aid in return for North Korea’s acquiescence to denuclearization — and we can also expect that Kim Jong-un will brush aside such a bargain. We can also expect Mr. Yoon to call for dialogue with the North,  offering to build on Mr. Moon’s three summits with Mr. Kim in addition to the three times President Trump met the man.

In the end, though, neither Mr. Moon nor Mr. Trump got anything beside avalanches of publicity for their attempts at reconciliation. North Korea is if anything more dangerous now than it ever was. Mr. Yoon has promised to get tough, to refuse to reward missile shots, much less another nuclear test, with hand-wringing pleas for talks, and he has said he won’t yield on sanctions or otherwise budge before the bully.

Mr. Yoon will have a hard time sticking to that policy while fending off those who share the dream of appeasement, including a fatuous end-of-war declaration affirming that the Korean War, which was interrupted in 1953 with a truce but not a peace treaty, is truly over. He will see this nonsense for what it is, a prelude to a treaty that would end the American-South Korean alliance and require the us to withdraw our GIs.

It’s hard not to imagine that the way the war in Ukraine erupted played a role in the political shift in Korea. It certainly is a moment to show strength and determination. Mr. Yoon will find Kim Jong-un a much more difficult case than Free Korea’s two former presidents. The best course for Korea’s new president is to stick to his campaign vows and show Mr. Kim — and the rest of the world — that he means business.