Ballistic Missile Defense

There are distinct kinds of nuclear threats, such as:

1. Attempts by nuclear superpowers to win a nuclear war in a first strike
2. Attempts by nuclear superpowers to immunize themselves from U.S. nuclear coercion by establishing a secure second-strike capability
3. Attempts by nuclear non-superpowers to immunize themselves from U.S. conventional military coercion by establishing a credible limited first-strike capability
   
4. Acts of desperation by actors with either no return address or with good bunkers and no regard for their own citizens

In other words, we have to distinguish between ABM as used in nuclear war-fighting, and ABM as an attempt to undo nuclear arms proliferation. I see the latter as futile. Regarding the former, I'm OK with a porous low-cost ABM effort that offers an alternative to launch-on-warning as a way to restore mutual assured destruction between two adversaries armed to the teeth with heavily-MIRVed ICBMs (10x like the old MX and SS-18). But it is futile to use ABM to 1) prevent China from acquiring effective MAD parity, or 2) neutralize the ability of a North Korea or Iran to threaten anybody with nuclear ballistic missiles. We have to accept that China can incinerate an unacceptable fraction of our West Coast, and that a country like North Korea can (via speedboat if necessary) get a nuke into some city that we don't want to lose.

To get decent coverage for a boost-phase defense would seem to require either a big investment in orbiting assets or almost a cordon around the adversary, who can cheaply increase defense porosity by e.g. spinning his boosters or deploying warheads and penetration aids earlier, perhaps even while the upper atmosphere still degrades directed-energy weapons. Once you get past boost phase, I suspect that the physics and economics are overwhelmingly on the side of offense.