On a cold afternoon in February, several former American officials hurried to the Hilton Hotel in Berlin, a city long known for its Cold War spies and intrigue. They had traveled there for a private meeting with senior representatives from North Korea, the most reclusive government in the world. Over the next two days, the Americans gathered in one of the hotel’s modern conference rooms and listened to a surprising new proposal. Supreme Leader Kim Jong Un, the North Koreans said, wanted to resume negotiations in hopes of ending decades of hostility between the two countries.