> Meanwhile, it is trivial for us to end their civilization. If we decided to, with more or less today’s technology, we could accelerate a chunk of tungsten to comical speeds and obliterate their homeworld.

We wipe out their planets, then the 99% of their civilisation that isn't living on planets wipes out our planets. Now we have two very angry civilisations (without planets) in a protracted interstellar war. Great.