In discussions between the U.S. and China about reining in North Korea, one topic remains taboo: What would happen if Kim Jong Un's regime collapses?

For years, China has rebuffed U.S. attempts to raise the topic at so-called Track 2 dialogue sessions between academics in each country's foreign policy establishment, according to Bonnie Glaser, senior adviser for Asia at the Center for Strategic & International Studies in Washington, who has led the U.S. side in such talks. Attendees included people from Chinese government-affiliated research institutions and military officers, she said.

From China's perspective, officially broaching the issue could alarm its neighbor, which has received Beijing's backing since the Korean War in the 1950s. There's also a fear that it would give the U.S. an advantage in one day reunifying the Korean Peninsula on its terms.