In the wake of threats from the North Korean government, the United States is sending batteries of missiles to Guam and a warship (the USS John S. McCain) that can shoot down rockets fired from North Korea. As powerful as these defenses are, their effectiveness may lie more in their symbolic value than in how well they would stop missiles.
"The real value as a deterrent is to show we're interested in the region," Philip Coyle III, a former associate director for national security in the White House Office of Science and Technology, said in an email.
"The Administration felt they had to do something to respond to the latest DPRK (North Korea) sword rattling, especially after North Korean officials mentioned Andersen Air Force Base in Yigo, Guam, among potential targets."
That isn't to say that the U.S. can't stop North Korean missiles. But missile defense is different depending on whether one is trying to stop a single ballistic missile or a barrage of them fired by North Korea. [North Korea Issues Hollow Threats Against U.S.]
Modern missile defense is nothing like the "Missile Command" video game, in which players defend cities from incoming ballistic missiles by blowing them up just before they strike their targets. In real life, targeting incoming missiles is a lot harder, and missiles still need to be pretty close to their targets to destroy them, and they don't always manage to do that. The systems that the U.S. sent to the waters off South Korea and to Guam (a U.S. territory) are "kinetic kill" designs, which means the missile is fired and actually hits another missile, ensuring complete destruction of the warhead.
There are two ways to take out missiles: close to the target when they are in the "descent" or "terminal" phase of their trajectories, and farther off during launch or before the missiles re-enter the atmosphere. (Any ballistic missile going more than about 200 miles traces a high, arcing path that takes it out into space, albeit briefly).
The first method is best for protecting specific targets such as military bases because by the time the missile re-enters the atmosphere there isn't as much time to stop it. It's also much clearer where it is going. The second is better for protecting larger areas -- it's the idea behind many missile defense systems proposed during the Cold War to stop intercontinental missiles from the USSR.
Guam will be defended by a system called THAAD, for Terminal High Altitude Area Defense. The missiles from a land-based THAAD battery can steer toward and track down ballistic missiles in the final stages of descent -- still up to about 150 miles from the target. They can protect targets such as air bases, but they aren't designed to defend large swaths of territory. And there's one problem with THAAD: It hasn't been used in real battle yet. However, in October the U.S. Missile Defense Agency reported a successful test in which THAAD missiles hit four out of five targets.
The second method is best for protecting large swaths of territory because it hits the missiles as they launch, or just as they exit the atmosphere, when there is more time to respond.
That's what the Aegis system does. It's the one that is on the destroyer USS John S. McCain. It has a larger defensive "footprint" than THAAD, meaning it can defend a larger area.
Aegis is designed to stop missiles when they are in "boost phase," shortly after launch, or before they re-enter the atmosphere after the engines are disengaged.
Both systems can work in concert, with THAAD using data from Aegis radar and satellites. And both systems use missiles that can maneuver to home in on attacking missiles.
Why not just use the Patriot system? Patriot missiles aren't really made for defending against longer-range weapons. The kind of missile a Patriot is most effective against has a range of up to about 600 miles. Such missiles aren't moving as quickly as those that can travel from the North Korea to Guam.