OK, let's say that you have an interstellar human population. You've got people who live on different planets in various systems, but you also have people living in space stations in different systems.
Now, given such a society, the ultimate weapon would be a long-range kinetic impactor: something that accelerates a reasonably-sized mass to a substantial percentage of the speed of light, so that it can hit planets and take them out. Eventually.
The obvious problem with such a weapon is that there's basically no "aiming" mechanism possible which is accurate enough across interstellar distances. Even if you knew the locations of all celestial bodies in the target system when you left and could predict their locations by the time the projectile arrives, you still cannot guarantee that you can aim with sufficient accuracy to hit the target location.
No problem; you just install a guidance system in the projectile. It should have some kind of camera that can see stuff, and it should have some minimal reaction mass so that it can perform course corrections based on what it sees.
Which brings me to the question. If you've got a projectile going 0.99c (relative to the target system), would relativistic effects make it difficult to perform the course-corrections needed to strike the target planet (assume it's Earth-sized)?