If MAD were all that you actually needed, NATO wouldn't exist. It's entirely reasonable to believe that France is not going to commit national nuclear suicide

If MAD were all that you actually needed, NATO wouldn't exist. It's entirely reasonable to believe that France is not going to commit national nuclear suicide merely to defend Czech or Poland from a Russian invasion. MAD - mutual assured suicide - would only be utilized in a case where their own existence were threatened. That's where a very powerful traditional military comes into play. That's why NATO exists instead of merely relying on having a stockpile of nukes to point at Russia. You might actually have to stand off Russia in a real military confrontation or at a minimium give the strong impression that you can if necessary. Russia can gamble that France isn't going to commit suicide via nuclear weapons to save Lithuania, and it would be a correct bet, at which point it's down to traditional military vs military.