MAD is predicated on the fact that ICBMs take time to arrive at their target, and are effectively unstoppable. The idea is that if nuclear state A was to target

MAD is predicated on the fact that ICBMs take time to arrive at their target, and are effectively unstoppable. The idea is that if nuclear state A was to target nuclear state B, B would be able to launch their own arsenal at A before B was obliterated. Additional methods of delivery such as sub nukes, strategic bombers, Carriers with nukes, etc. further reinforce the concept as B would be able to retaliate even after its destruction. Now, the modern context has weakened MAD somewhat, because it's strictly an interpretation of nation-state level nuclear conflict. With the proliferation of (plausibly deniable) non-state actors in the 21st century, the concept breaks down as it makes it much harder to 'attribute' a target to blanket nuke in response.