The problem with that is an old doctrine called MAD, or Mutually Assured Destruction. It's the cornerstone of nuclear weapons capability and those weapons' purpose of never being used. If NATO was to attack Russia and give Putin reason to believe that his regime or Russian sovereignty was under threat, they would launch their weapons. That's not my opinion, that's Russian doctrine. This would immediately trigger counterattacks from the US and the UK, most likely also France, which would then most likely trigger preemptive attacks from China and North Korea, defending their interests in the Pacific and South East Asia. Yes, a NATO army could march into Moscow by tomorrow, but there's a very good reason they won't do it.