A new era of modern warfare began when German V-2 missiles hit London in September 1944. Within the next few years, the U.S. military was issuing reports on how to proceed in the development of defensive counters to ballistic missiles.
Anti-ballistic missile (ABM) defense, as originally conceived in the 1950s and 1960s, was a Cold War era answer to the nightmare of Mutually Assured Destruction, the linchpin of Soviet and American deterrence.
American thinking about missile defense has been incoherent from the very beginning. The issue is superficially simple: the Soviet Union threatened the American people with nuclear missiles, so the U.S. should naturally have tried to defend them against those missiles.