AP Photo

At this moment the Kang Nam, a North Korea tramp freighter, is on the high seas tailed by a team of American destroyers and submarines and watched by reconnaissance satellites and aircraft. On board, its cargo could be plutonium pellets, missile parts or semi-ripe melons. In any event, Washington wants to know what is in the rusty ship's hold.

Why the interest in this particular vessel? The Kang Nam is a "repeat offender" and known to carry "proliferation materials." As an unnamed American official told Fox News this month, "This ship is presumed to be carrying something illicit given its past history." United Nations Security Council Resolution 1874, unanimously passed on June 12, broadened the concept of illicit cargoes as far as North Korea is concerned. It prohibits...