Mr. Moon clearly understands that Mr. Kim’s economic ambitions are the key to keeping the diplomatic process going. On his visit to Pyongyang this week, Mr. Moon brought the heads of state-backed rail and energy corporations, along with the CEOs of South Korea’s top conglomerates. No deals were struck, and the group has been mum so far about its meetings. But its presence was enough to send the message that South Korea stands ready to move forward with major economic cooperation projects with the North.

In one astonishing scene, Mr. Moon addressed a crowd of 150,000 cheering North Koreans and pledged to “hasten a future of common prosperity.” He praised Pyongyang’s “remarkable progress” and said he understood “what kind of country Chairman Kim and his compatriots in the North want to build.” In a formal agreement known as the Pyongyang Declaration, the two leaders committed to reconnect rail and road links between the two countries, reopen a frozen joint industrial zone in Kaesong and a tourist site at Mount Kumgang, and make plans for a special economic zone, of the kind Deng promoted to open up China to foreign investment in the 1980s.