Struggling under heavy international sanctions imposed after Tiananmen, China embarked on economic reforms to break the impasse. One piece of this puzzle involved normalizing diplomatic relations in 1992 with South Korea, the bustling economy across the Yellow Sea.

But that move rattled China's traditional partner North Korea, which believed it had a "friendship cemented in blood" since fighting side by side in the Korean War. China teaming with the enemy to the south represented a betrayal in the eyes of Pyongyang.

Sensing a threat to its security, North Korea concluded that developing nuclear weapons was the only way to ensure its survival, despite the global opposition such a move would bring. Pyongyang's nuclear program at the center of the U.S.-North Korean dialogue today is rooted in that reversal by China toward South Korea.