•North Korea's threat to launch four intermediate-range ballistic missiles into the ocean near Guam could mark the first combat test of the sophisticated missile defence systems of the United States and its Asian allies.
If the Hwasong-12 missiles do make it off the ground, the options for stopping them mostly rely on hitting them on the way down in their "terminal" phase.
Here's a look at the three phases of the trajectory:
ON THE WAY UP - 'BOOST PHASE'
The Hwasong-12, a domestically developed liquid-fuelled missile, has a maximum range of 4,828km and hits an altitude of about 756km on the way to its destination.
The velocities needed for those numbers mean that by the time the missile has been in the air for one minute, it is already travelling at several times the speed of sound.
A missile trying to chase and hit it from behind would have no chance during this part of the flight.
The Terminal High Altitude Area Defence (Thaad) system, several units of which are now stationed in South Korea, could use its radar to track the launches. But it is not designed to hit them as they climb.
IN MIDFLIGHT - 'MIDCOURSE'
Once the booster burns out and it reaches the edges of the earth's atmosphere, it is no longer accelerating, but is travelling blisteringly fast and higher than some satellites.
This phase is the most difficult time for an interception because a fast-moving warhead can also release decoy balloons.
But destroying the ballistic missile in space is attractive because it keeps high-speed debris and explosions far from the friendly target.
Japan and the US have ships equipped with SM-3 missiles that can hit intermediate-range missiles in midcourse. But the trajectory of a Hwasong-12 aimed towards Guam could put the midcourse portion too far past the Sea of Japan, where Japan's ships carrying SM-3s are usually stationed.
The existing US system is based in Alaska and California, and is not in a position to stop a missile flying that far south over the Pacific.
ON THE WAY DOWN - 'TERMINAL PHASE'
The warhead falls towards its target in this phase. Some missiles, like the retired Pershing II, can steer during this stage.
But that is not true for 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 as the SM-3 can intercept a ballistic missile at this point.
The US does not typically disclose the exact positions of its warships, but several Arleigh Burke-class destroyers, which are equipped with SM-3s, are based in the western Pacific. If they were stationed near Guam, they could take a shot at the Hwasong-12s.
The Thaad system, of which at least one is permanently deployed at the US airbase in Guam, could also take a shot.
The air base may also 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.