Russia’s nuclear sabre rattling isn’t credible because their nuclear arsenal is not dispersed. Dispersal is the key to a nuclear exchange and it’s why both sides spend so many billions on submarines and stratobombers. Since the cold war Russia’s nuclear assets have been sitting grouped at bases around the country. If Putin wants to be credible he has to first order the planes in the air and the submarines to sea. But the submarine he sent to Cuba was accompanied by a tug “just in case.”