Answer:
MAD, or mutually assured destruction, means that if one destroys the other, then the other side will be destroyed too.
Explanation:
This was the entire concept of the cold war. The arms race was so that one side could destroy the other more than they could do to them. Essentially, you wanted your punch to hit harder than theirs. MAD made it so that if either side fired something, then both sides would lose. The destruction is mutual.