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