The ballistic missile defence system is a U.S. system only. It was not built to go against Russia or China. It was built to go against the terrorist threat of North Korea, mainly.
It has missile fields. It has radar. Right now, it launches and hits a bullet with a bullet. The system works, and in anger, it could also defend against an attack from Russia, obviously, because a missile is a missile. It doesn't matter what it is.
The problem is the number of missiles available to achieve an effective hit. It's about shot doctrine. You launch so many missiles against one missile and it blows up in outer space.