Unlike with nukes (Project Orion notwithstanding), having access to relativistic weapons also implies having access to space travel. This is not necessarily the

Unlike with nukes (Project Orion notwithstanding), having access to relativistic weapons also implies having access to space travel. This is not necessarily the case, especially given biological constraints, but let's assume you have space travel. Well, then you are also spread out, and given enough time, you spread throughout a galaxy. With that level of spread, the safety in numbers and distribution over a large space is a form of strong defense against relativistic weapons from other civilizations. Furthermore, the technology for these weapons may also open up avenues for defense, even if such is not the case for nuclear weapons. Consider a relativistic missile detection system that continually scans for incoming missiles and is able to quickly accelerate its own for head-on collisions. Granted, there's not much notice. But let's say you have missiles at 0.99c - quite fast - and a light year away (would any closer be plausible for a launch? Maybe with relativistic ICBMs... or ISBMs rather). By my non-relativistic calculations, that gives you almost 90 hours to prepare countermeasures. A "simpler" countermeasure is just to go overboard with the spreading idea. Simply live on millions of asteroids with your advanced propulsion tech slapped on. When a relativistic threat is detected, each asteroid is nimble enough to get out of the way, and if some are still hit, it's a small loss. Though, the attacker could just shoot many smaller, faster missiles dispersed over a wide area. Not civilization-annihilating now, unless it was overwhelming in number, but still painful.