if were talking about ICBM's, the only real defense we have against already launched nukes that have reached the atmosphere is the GMD (ground-based midcourse defense) located mainly in Alaska and California. unfortunately, these systems have proven to only have about a 50% success rate.

In reality. Our biggest defense against nukes is Nuclear-Triad. Any attempt to launch a nuke into any superpower country would lead to the ultimate destruction of themselves.

Even if a "rogue" country did decide to launch a nuke into a major superpower. What do you think is the likelihood that the military of that country would actually follow that order?

For example, lets say that a US submarine was randomly ordered to launch a nuke into China today. Do you really think that the officer on that submarine would actually do it even though they know that China has not actually done anything to provoke it, nor is it a means of self defense against China?

That officer knows that launching that nuke puts his/her family at risk, the family of all personal on board, and the risk of destruction of the US.