Call it a spring thaw. After a year of escalating tensions over North Korea’s nuclear program, the announcement this week that the country’s leader, Kim Jong-un, is willing to start negotiations with the United States on ending the program is a welcome surprise, if one being met with caution. After all, it was barely two months ago that Mr. Kim and President Trump were fighting over who has the bigger nuclear button. On Tuesday, Mr. Trump expressed cautious optimism about the change in direction, calling it “possible progress” that also “may be false hope.” A pair of South Korean officials left Seoul on Thursday for Washington, where they’ll try to gain his support. Barring any unforeseen events, the next step would be a meeting between Kim and South Korean President Moon Jae-in at a border village in late April. There are no doubt many “critical moments,” as Mr. Moon said on Thursday, “before reaching the denuclearization of the Korean Peninsula and a permanent peace.” And there are plenty of reasons to question how sincere North Korea is about its offer. Talks with Pyongyang have repeatedly fallen apart, and some observers suspect its recent grand gestures are merely a bid for time. But there are also reasons to be hopeful, given all that’s happened over the past few weeks. Here’s a look back at some of the critical moments that got us here.