I'm aware of the various missile defense systems we've developed. None have demonstrated a consistent ability to shoot down ICBMs.

We've had more luck with short and medium range defense, but that's not really what we're talking about here.

With ICBMs, you either have to shoot it down during boost, which means you more or less have to be on station waiting for the launch, or you have to shoot it down midcourse, when it's going 15,000mph a hundred miles above the planet.

They talk about terminal defense, but I'll believe that when I see it.

The closest thing we have to a real defense capability is the GMD, which claims a "50% success rate" against individually launched missiles, with advanced preparation and warning, and a couple more caveats besides. It's nothing close to a real world demonstration. At best it's a last ditch Hail-Mary strategy.

The simple fact is that it's really, really hard to shoot down something moving that fast. A bullet is standing still by comparison. And even if we could that doesn't solve for hypersonic cruise missiles, stealth bombers, or any of a half dozen other deployment strategies.

If we did develop truly reliable ballistic missile defense, it would probably be a bad thing, as our adversaries would presumably not be too far behind, and we'd enter a world in which nukes could be deployed through other means without the risk of being immediately blanketed by ICBMs in response.