Generally I agree with this analysis, but I think there is an attack that would work:
Strap small N-bombs (or conventional bombs?) to as many as asteroids as possible, launch them at the target area, and then blow them up ~500,000km away. Suddenly you have thousands (millions?) of significant chunks of rock travelling at 0.5c and spreading out. If you got the explosion point right and had enough asteroids you could cover a reasonable amount of the likely positions, even based on random movements.
It's unlikely to destroy an entire asteroid-belt based civilization, but wouldn't be trivial to evade either.
Strap small N-bombs (or conventional bombs?) to as many as asteroids as possible, launch them at the target area, and then blow them up ~500,000km away. Suddenly you have thousands (millions?) of significant chunks of rock travelling at 0.5c and spreading out. If you got the explosion point right and had enough asteroids you could cover a reasonable amount of the likely positions, even based on random movements.
It's unlikely to destroy an entire asteroid-belt based civilization, but wouldn't be trivial to evade either.