First strike

In nuclear strategy, first strike capability is a country's ability to defeat another nuclear power by destroying its arsenal to the point where the attacking country can survive the weakened retaliation.

One reason critics oppose missile defense systems such as Reagan's proposed Strategic Defense Initiative is that they view them as undermining one of the fundamental premises of mutual assured destruction: The proposed defense systems, while intended to lessen the risk of devastating nuclear war, would in fact lead to it. The other side, seeing that a nation was building a defense against a first strike and believing that the other could launch a first strike if it dared, would then launch a pre-emptive first strike while they still had a chance. The reasoning being that mutual destruction is better than defeat.