There was a treaty: https://en.wikipedia.org/wiki/Anti-Ballistic_Missile_Treaty https://en.wikipedia.org/wiki/Anti-Ballistic_Missile_Treaty Summary: It was ok

There was a treaty: https://en.wikipedia.org/wiki/Anti-Ballistic_Missile_Treaty https://en.wikipedia.org/wiki/Anti-Ballistic_Missile_Treaty Summary: It was okay for each side to have a limited ABM capability but not an extensive one. Treaty was exited by USA side in 2002 and the stated reason was that ABM-like defense would be needed against "rogue states".