When North Korea launched its Kwangmyongsong-4 satellite into space in February last year, officials heralded the event as a birthday gift for dead leader Kim Jong Il. But the day also brought an unexpected prize for the country’s adversaries: priceless intelligence in the form of rocket parts that fell into the Yellow Sea.

Entire sections of booster rocket were snagged by South Korea’s navy and then scrutinized by international weapons experts for clues about the state of North Korea’s missile program. Along with motor parts and wiring, investigators discerned a pattern. Many key components were foreign-made, acquired from businesses based in China.

The trove “demonstrates the continuing critical importance of high-end, foreign-sourced components” in building the missiles North Korea uses to threaten its neighbors, a U.N. expert team concluded in a report released last month. When U.N. officials contacted the implicated Chinese firms to ask about the parts, the report said, they received only silence. […]