I think a point everyone misses in saying that if we had just quietly left Cappella the Shivans wouldn't have bothered us is the fact that they kept hitting the convoy's leaving the system. If they really didn't care about us, then they only would have killed what was absolutely necessary to achieve their goal of blowing up the star, but they didn't. They went after all the civilian convoys with huge waves of fighters and bombers, which even given the Shivan armada seems like a waste resources. If ants were invading your house, and you stomped a few big ones and then they all turned and left, you wouldn't bother to chase after them.