a Cold War
era concept, meaning a peculiar sort of peace, or rather of lack of war.
The idea was that the US
and the USSR
hated each other's guts, and the only reason why they did not blow each other to tiny bits was that they were convinced that they could not win with a decisive first strike
Basically, we have been kept alive for many years by fear of retaliation.
In this context, it becomes clear why an efficient, nationwide Anti Balistic Missile system was a factor of instability in the Cold War world: if opponent A thinks that his defense system can efficiently intercept the enemy's counterstrike, A may well be tempted to unleash a first strike.
Worse than that: the very idea that A is designing/building an efficient defence system may prompt B to do a preemptive strike before A completes the defence system and succumbs to temptation.
I always wondered why we that lived in a NATO country in Europe did not just go insane with fear during the Cold War years. I certainly was afraid.