With U.S. diplomat Robert King headed to Pyongyang this week, the focus turns from North Korea's rapprochement with its southern neighbor to its still-tense relationship with the United States. There's reason to be optimistic that trans-Pacific relations will improve, but many roadblocks still remain.

In recent weeks, North Korea has launched a full-scale charm offensive with the South, striking a deal on family reunions and agreeing to reopen a joint industrial zone. All the while, the North has issued official statements...