On Oct. 11, the Lighthouse Winmore departed South Korea, supposedly heading for Taiwan. But eight days later, a U.S. spy plane photographed the ship unloading oil in the East China Sea. Receiving the oil was a smaller vessel — the Sam Jong 2 — which was flying North Korea’s flag. U.S. authorities say 600 tons of oil were unloaded, breaking U.N. sanctions on North Korea. The Lighthouse Winmore was later impounded by South Korea. And Taiwan arrested Chen Shih-hsien, a fishing magnate who leased the ship. Mr. Chen also owns two other ships. And officials say that one of these — the Billions 18 — also smuggled oil. U.S. spy planes photographed several other ships suspected of abetting North Korea. But the opaque world of global shipping means that smuggling will remain difficult to detect.