An episode involving missile defence* from the West Wing holds up very well today. The craggy old American chief of staff is in favour, out of fear of what rogue regimes might do to America. The British ambassador is opposed because it’s impractical, violates international law, and risks worsening the global nuclear weapons situation.
I can see why people like the idea of being able to stop a few missiles launched by North Korea or Iran, or by a rogue commander somewhere. At the same time, I think the dangers of a nuclear arms race make the development and deployment of such a system unwise, even if the major technological hurdles could be overcome. It’s the classic security dilemma: you build something meant to make you safer, potential opponents interpret it as making them less safe (by reducing the credibility of their deterrent) so they build expensive countermeasures. In the end, everyone has wasted money on the race and everyone ends up less safe. It could also tempt decision-makers into recklessness, based on false confidence that the system will nullify any response to their aggression.
We should be working to de-alert and dismantle the nuclear arsenals of the authorized nuclear powers under the Non-Proliferation Treaty. Against that backdrop, resisting proliferation to new nuclear states would be more plausible.
* I don’t mean defending things like aircraft carriers from ballistic missiles. I mean systems to protect domestically-located military facilities and population centres from ballistic missile attack, probably with nuclear weapons.