[Saxon]

Now, you know the enemy has this horribly huge and powerful planet coming your way. How do you stop him?

It seems to me, if we can turn a big sector of asteroids into a planet of some stability, we should be able to turn that machine into a weapon. Sure, that mobile ring world is big, but if I can force asteroids together to form a planet, I can put pressure on the outside of your ring world, which is already being pulled in by the star’s gravity, and watch the whole thing crumble.

Ok, you have some defenses, but I will have a number of ships going after you and, for good measure, I will have puts some engines on a few cast off moons and will drive them straight at you. A moon is much more maneuverable than that big old ring world, so I should be able to get it up to some near relativistic speeds and then have it shatter before it hits, really spreading the impact out over your surface. Sure, you can shoot the moon, but that doesn’t bother me, even if you get the engines, the mass and velocity are still there. Niven and Pournelle love this trick.

Now, as we are getting far fetched, why would I bother with a moon? After all, you figured out how to move a sun, so why can’t I? Again, one sun is more maneuverable than a sun and a ringworld, so I am going to chase you down with a mobile sun. I will just leave one sitting outside the warp point I expect you at and when you warp in, I will accelerate more quickly than you.

Then, my mobile sun heads out to your home system. As someone pointed out, the only way to make a sun go away is to detonate it, which will destroy your system. You either detonate the sun, destroying the system, or I run over your planets with it, destroying the system. Starting to look like Mutually Assured Destruction again.