But the trajectory of a Hwasong-12 aimed toward Guam could put the midcourse portion of its flight too far past the Sea of Japan, where the country’s ships carrying SM-3s are usually stationed. Intercepting the North Korean missile at this point would be difficult without moving those ships closer to Guam.

America’s existing system to stop intercontinental ballistic missiles aimed at the continental United States is called Ground-Based Midcourse Defense. It is based in Alaska and California, and is not in a position to stop a missile flying that far south over the Pacific. It also has a spotty record, although its most recent test, in May, was a success.

On the Way Down

As with all ballistic missiles, gravity takes over after the midcourse phase, and the warhead falls toward its target. Some missiles, like the retired Pershing II, can steer during this part of the flight, called the “terminal phase”; that is not true of the Hwasong-12, whose final destination is determined entirely by course corrections when its engines are still running.

Theoretically, that makes it an easier target.

The SM-3 can intercept a ballistic missile at this point. The United States Navy does not typically disclose the exact positions of its warships, but several Arleigh Burke-class destroyers, which are equipped with SM-3s, are permanently based in the western Pacific. If they were stationed near Guam, they could take a shot at the Hwasong-12s. It’s unclear whether Japan would reposition any of its warships to defend the area around Guam.

The Thaad system, of which at least one is permanently based at Andersen Air Force Base in Guam, could also take a shot. Thaad has a good record in tests, most recently destroying a target simulating a missile similar to the Hwasong-12 in July. Finally, the air base may be defended by older, shorter-range Patriot missile batteries, the most advanced of which — the Patriot PAC-3 — can also shoot down slower ballistic missiles.

Worth Taking a Shot?

Missile defense is an attractive but tricky strategy in dealing with missile threats. It has been compared to hitting a bullet with a bullet, and even just testing such systems can be expensive. The payoff, of course, is protection from enemy ballistic missiles, which proved deadly in the first gulf war despite attempts to shoot them down.