A National Missile Defense
or its smaller-scale counterpart, a Theater Missile Defense
can itself be considered a weapon of nuclear terrorism
, albeit terrorism performed by a major world power
. As such it will not be called terrorism and, instead, will be referred to as "foreign policy
An NMD allows a nuclear power, such as the United States, to have a credible threat of first strike nuclear attack in its bag of diplomatic tricks. Until recently, the US could not realistically threaten a rival nuclear power (only the former Soviet Union really filled this role) with first strike nuclear attack because of the problem of Mutually Assured Destruction, i.e. that both sides in a full scale nuclear war would be utterly annihilated, and therefore full scale nuclear war is illogical and extremely unlikely.
An NMD would, theoretically, allow a full scale first strike on a rival power while eliminating the rival's ability to retaliate. The balance of power is lost.
This is one of the reasons China and Russia are very unhappy about American plans to build NMDs and TMDs. Construction of such systems will allow the US to once again hold the world hostage with its nuclear weapons. China only has about 18 nuclear-equipped ICBMs (or so I've read), so you can be sure that construction of an American NMD/TMD will spark a massive nuclear weapons construction binge in China, as their only strategy to defeat such a defense system would be to overwhelm it with numbers.
The National Missile Defense is an aggressive tool of hegemony which will only succeed in sparking a new arms race and fattening the pockets of defense industry contractors. It is nuclear terrorism.