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.

The timing was significant. A month earlier, the U.S. had agreed to talks to formally end the Korean War, but that effort collapsed when Washington demanded the North’s nuclear weapons program be part of the discussions. A few days later, the Hermit Kingdom, officially known as the Democratic People’s Republic of Korea (DPRK), set off what it claimed was a hydrogen bomb at an underground site in the country’s rugged northeastern mountains. That nuclear test, the country’s fourth, left U.S. officials scrambling for new ways to deal with the threat from one of the world’s last communist regimes.

After the Berlin meeting, the former U.S. officials promptly returned to Washington to report to the White House. Sitting at a conference table in the Situation Room, they told the president’s top national security advisers that Pyongyang was prepared to stop testing nuclear weapons for a year. In exchange, the U.S. and South Korea would have to suspend their annual joint military exercises that the DPRK found provocative.