That's easy. Russia does not have military air or naval bases owned or leased in more than 100 nations around the world. The US would annihilate Russia in a nuclear war. Russia could destroy a few major cities perhaps with ICBMs. But the US has an incredible arsenal of submarine launch platforms, heavy bombers capable of delivering a nuclear payload stationed all around Russia, nuclear launch sites in Europe and missile defense systems. That war hasn't happened yet, because it's already been won by the US.