“In the nuclear age, the line between offensive and defensive weapons is blurred. Effective defensive weapons make a nuclear first strike thinkable.”
It is actually the exact opposite, which is why we pursue a MAD (Mutual Assured Destruction) policy. Yes, we have anti-missile systems that were developed to intercept incoming nuclear missiles, but those would be pretty useless against a large and sustained nuclear attack. With MAD, however, the promise of retaliatory obliteration has proved — and continues to prove — very effective. In a nutshell, MAD tells anyone that attacks us with nukes that they will cease to exist.
Then it’s not the opposite. A new defensive system, like SDI “Star Wars” will be feared as opening the door to a first strike. Ditto for positioning “defensive” missiles on an adversary’s doorstep.
If that seems unrealistic, some would argue that the same was true of Russian missiles in Cuba.