[PvK]

Ya I hardly think every empire would start with a relativistic cannon by default. Also, if you have a shell travelling at .9C, would it help if it were uranium? Maybe it would trigger a fission reaction on impact...

In any case, my point is that anti-ship weapons would not I think generally be effective as tools of genocide from space. It doesn't really fit my imagination to have single shots eliminating millions of civillians at once, the way they currently do.

If the ship weapons are supposed to be as strong or stronger than thermonuclear bombs, then how does any ship ever survive even a single hit?

I can appreciate that some sci-fi weapons could do massive damage to planetary populations, but I don't think all of them would, and many I don't think would be capable of causing casualties in the millions at all, though the same weapons might still be quite good at neutralizing weapons platforms...

PvK