Last year, North Korean leader Kim Jong Un and U.S. President Donald Trump were hurling kindergarten insults at each other — "Rocket Man is on a suicide mission," said Trump of Kim; "mentally deranged U.S. dotard," Kim retorted — while threatening to reduce East Asia to a post-atomic wasteland. Now, in a stunning and dramatic development, the two are to meet by May. Kim reportedly is willing to denuclearize and eager to talk directly to Trump, who has agreed.

But optimism about this turn of events must be tempered with cautious realism. North Korea is the nuclear problem from hell. Neither South Korea nor the United States can control the narrative; definitions of success or failure are highly relative; and Trump must enter the talks with no exit strategy. The six decades since the Korean War ended in 1953 — with a ceasefire but no peace agreement — have hardened an increasingly dangerous stalemate. Although neither side is likely to launch a premeditated nuclear attack, the risk of war from miscommunication, misperception or miscalculation is real.

All key announcements so far have come from Seoul, not Pyongyang or Washington. President Moon Jae-in, the son of refugees from North Korea, was elected on the promise of a two-track approach to the North: sanctions and diplomacy. This led to the Olympic initiative whereby Kim's sister, Kim Yo Jong, attended the Winter Games in Pyeongchang and the two countries competed as one team. Afterward, Moon's national security adviser, Chung Eui-yong, and intelligence chief Suh Hoon traveled to Pyongyang and Washington, where, standing on the White House lawn with Cho Yoon-je, South Korea's ambassador to the U.S. — but with no U.S. officials present — they announced the summit.