We were sufficiently ahead of the Russians both in technology and production that while the most prominent "leg" of their nuclear triad (land-based missiles, ocean-based missiles, and bombers) as the first, ours was the second. The Russian nuclear capability was vulnerable in a way our hard-to-find submarine fleet was not.
Now, it seems, the Chinese are making a concerted effort to develop a submarine-based nuclear threat. Such a Chinese capability would be a strategic game-changer.
We often make the mistake of thinking that because we are the pre-eminent military and above all nuclear power on Earth, we stand alone. We don't. The strategic threat the Chinese pose should no more be discounted than their economic power and conventional military capabilities.