The primary purpose of the US/NATO missile defense system is to intercept a small number of incoming from a rogue state (Iran and North Korea come to mind). It was never planned to intercept hundreds (or more) of warheads from a major Russian strike.
Edited to add that I'm speaking of the current missile defense - not the Star Wars program proposed by Regan back in the 1980's
Last edited by tdracer; 3rd Mar 2018 at 02:21.