USS George Washington a tempting target
Joint U.S. – South Korea naval exercises are underway in the Sea of Japan. North Korea has, true to form, threatened to retaliate for this "provocation", hinting at a nuclear attack.
North Korea probably isn't capable of nuking underway naval vessels but could conceivably attack a target within South Korea. This would risk North Korea ending up a smouldering, glow-in-the-dark wasteland.
The North could, however, attempt a stealthy submarine penetration of anti-submarine defences for a torpedo shot on the USS George Washington. The U.S. is in no position to respond militarily to such a provocation and the North knows it.
Update: More here.