It is highly unlikely that these are simply a response top North Korea’s bellicose rhetoric, its declared abnegation of the 1953 Armistice Agreement, and its ongoing rocket and nuclear programs. First, North Korea only has two sites from which it can launch its Taepodong/Unha SLV (or ICBM), and the liquid-fueled first stage means these cannot be launched quickly. They need at least a few days for assembly and fueling, most of which happens in the open, clearly visible to U.S. and allied satellites, and thus vulnerable to destruction on the ground via cruise missiles or air-launched missiles or bombs.
North Korea’s showcased road-mobile ICBM, also cited by Hagel, has yet to be tested (and there were considerations that the model on display was little more than a mock-up). Even the most pessimistic U.S. estimates put North Korea’s ability to strike the United States with a ballistic missile (nuclear tipped or not) at least two or three years down the road and only after several other tests of both the launch phase, and of reentry. While this could coincide with the completion of the deployment of additional interceptors, even without the additional units (assuming the ABM system works at all), the need for an additional 14 units to the existing 26 in Alaska and 4 in California seems unnecessary.
As noted above, North Korea still has only two sites capable of launching the Taepodong/Unha, and even if they were able to get a single missile launched from each (which would be intercepted by the existing U.S. ABM systems in an ideal world), the sites would be destroyed in a counter-strike before any second launch could take place. in other words, the 26 ABM-missiles should be sufficient for the two North Korean ICBMs, and perhaps for the half dozen or so still untested road-mobile missiles Pyongyang may or may not possess.
As a North Korean attack on the US homeland would lead inexorably (and rather rapidly) into a full-scale war, the U.S. retaliation, superiority in air-power, in cruise missiles (air, sea or sub-sea launched), in ballistic missiles, and in nuclear weapons (if the North for some reason tried to carry out a first-strike nuclear attack against the united States), would pretty much ensure that the north would not get a second chance to fire on the U.S. mainland. The current number of ABM missiles, then, are sufficient for a North Korean scenario.
BUT... although there is a bit of a political message to the North (don’t think you really have a deterrent, we [Washington] have sufficient resources to eliminate the effectiveness of your deterrent, leaving us still with a comfortable first-strike conventional option), the main signal may be for China. The united States sees beijing not only as the North Korean life-support system, but also as wielding considerable political influence in Pyongyang, no matter how much Beijing protests to the contrary. China (and Russia) see the shift in U.S. attention to the ABM system as aimed clearly at them, not at North Korea or Iran. The Pentagon made it clear that any shift in funding for the new ABM interceptors will not cut funding for the deployment of land and sea based systems for Europe, much to Moscow’s chagrin.
With the announcement of new interceptors, Washington is putting Beijing on notice, both for its increasingly assertive action in the East and South China Seas and for its facilitation of North Korea’s threatening behavior. China argues that it cannot seriously clamp down on North Korea economically or politically for fear of triggering a refugee crisis on its border, but Washington believes Beijing encourages North Korea’s aggressive posturing, to try and gain politically leverage of its own with the United States, South Korea and Japan in return for Chinese-sponsored dialogue with the North.
Washington is now letting China know that Beijing will find its own interests undermined by North Korea’s behavior. And this will not be a short-term response, like keeping U.S. ships and submarines in the region a few weeks after exercises with South Korea conclude. The U.S. expects the Chinese to change North Korean behavior (and their own in the regional territorial disputes), and the new interceptors reference China’s strategic position, more so than North Korea’s. It will be interesting to monitor China’s response to the interceptors, both in its public reaction to Washington and in its private actions toward Pyongyang.