As the United States and South Korea began their annual “Foal Eagle” joint training exercise this past week, North Korea conducted an exercise of its own. Four medium-range ballistic missiles were fired from North Korea’s west coast in a simultaneous volley, and three of them ultimately landed just inside Japan’s Air Defense Identification Zone a thousand kilometers east. No new capabilities were demonstrated – this was a salvo launch of four Extended-Range Scud missiles, very similar to a three-rocket salvo  last September. North Korea has captured public attention with tests of new missiles on several recent occasions; this was a reminder that their existing missiles are already quite capable.
There was a great deal of confused speculation in the immediate aftermath of the launch, as the launch site was at or near North Korea’s Sohae Satellite Launching Station. That facility is normally associated with much larger rockets, and we were left to wonder whether they might have slipped an early test of one of their intercontinental ballistic missile prototypes in with three ordinary Scuds. The trajectory was never a good match for an ICBM test, however, and North Korea ultimately released a video of the launch that confirmed what we had come to suspect. The test took place from very near the Sohae facility, but appears to have been conducted by an operational military unit using Scud-ER missiles on mobile launchers. The use of the Sohae facility may have been misdirection, or it may have reflected a desire to use that facility’s radars for tracking.
The Scud-ER has been in service for at least a decade, though it was not well understood in the West until last year. It uses heritage Scud motors and presumably guidance systems with a new airframe incorporating larger propellant tanks, and can deliver warheads to a range of just over 1000 kilometers. Smaller and cheaper than North Korea’s other medium-range missiles, it can be launched from the same TELs as ordinary Scud missiles. The new Pukgugsong-2  solid-propellant missile, when it enters service, will be more survivable, but for now the Scud-ER is about the best weapon North Korea has for conducting strikes against strategic targets in South Korea and Japan.
This was probably the point of the latest exercise. Foal Eagle is a training exercise aimed at maintaining and demonstrating the ability of the US-ROK alliance to wage war against North Korea. They’ve just demonstrated that they can wage war right back, with weapons they have in operational service today. And the trajectory also carried a not-very-subtle message to Japan: that North Korea understands the role Japanese ports and airfields play in allied plans for war on the Korean peninsula, and that Japan will be a battleground if those plans are carried out.
The four-rocket salvo was likely intended to demonstrate the ability to saturate allied missile defenses, such as the THAAD anti-missile defense system that the US recently started to deploy to South Korea. THAAD, in conjunction with the existing Patriot system, should have no difficulty stopping a four-missile salvo at a high-value target; it has successfully stopped a five-missile salvo in tests. But four missiles at once almost certainly isn’t the limit of North Korea’s capabilities, nor is it in their interest for us to know what those limits are. We don’t know how many missiles they can launch simultaneously, or from how many sites. They don’t know how well our missile defenses can deal with large salvos. Even we are not absolutely certain.
That’s the headache North Korea has given allied planners on the eve of Foal Eagle: whatever war plans we may conceive and practice, they will confound with results that nobody can predict. Key military bases and logistics facilities might suddenly be contaminated with half a ton of VX nerve gas, or destroyed by nuclear fire. The Japanese government might ultimately balk at playing their role at those stakes. Or perhaps our missile defenses will stop even the largest volley the North can fire, and our counterattack might destroy all their launchers before they can conduct a second strike. Maybe. But Pyongyang doesn’t need sophisticated new weapons to confront us with the sort of risk no one will be eager to take; their old ones still work just fine.