South Korean President Moon Jae-in’s visit to Washington Thursday to meet with U.S. President Donald Trump marks the most significant moment in North Korea diplomacy since the immediate aftermath of the Hanoi summit.

Since President Trump and Kim Jong Un walked away from Hanoi with no deal, momentum in U.S.-North Korea diplomacy has ground to a halt.

The lack of an outcome at first appeared to reflect a simple and understandable disagreement over the extent of sanctions relief.

But comments by National Security Advisor John Bolton and other U