Missile defense is a dicey topic. In the 1980s the Soviet Union realized that they could not afford their own anti-missile systems, or make their ICBMs resistant to US anti-missile systems. This was partly responsible for increased tensions as the Soviet Union faced an existential threat of losing a nuclear war.

They responded by participating in arms reduction treaties which had the same effect as the US building an anti-missile system which could intercept 95% of Russian weapons. If the US built a full missile defense umbrella, adversaries would respond by building enough weapons to defeat the umbrella.