Does MAD still apply?

[Feanor]

That is a good question and the answer lies at the core of MAD theory, that is to have an ASSURED 2ND STRIKE CAPABILITY. Since nuclear submarines are immune to land attacks and first strikes and their movements are unpredictable they are the best tool to assure a 2nd strike (response) and hence assure mutual destruction.

Or a good early warning capability that allows a response to a nuclear attack, before one's own assets have been struck.

 

[SABRE]

Or a good early warning capability that allows a response to a nuclear attack, before one's own assets have been struck.

That would depend on the availability of reaction time. The geographical distance between USA and Russia/China is far enough to make early warning viable for quick response. But the scenario between Pakistan-India or China-India is very different since they have contiguous borders. Reaction time is very short. Assured 2nd strike will have to depend on nuke subs.