There wont be any active war between US and Russia, because it would be just M.A.D.

The US and Russia are the only nations that have a nuclear triad - that means they have sea (submarines and other ships with rocket silos), land (silos and land based vehicles with a rocket launching system)
and air (strategic bombers) based launching systems for nuclear weapons.

It is not possible for either of them to destroy the following nuclear counter attack with a massive scale nuclear first strike.

Even nowadays 26 years after the cold war the US and Russia have active nuclear forces, which are always on alert and ready for a nuclear strike.

Btw. if it should come to a showdown my bets are on US+NATO+ASEAN+Japan+South Korea+Israel vs Russia+China+Iran+North Korea ; India vs Pakistan solo match ; South America and Africa will probaly just watch;
not sure where to put turkey and the saudis.


*edit

i'm pretty sure there exist a counterpart for this in the US: Dead Hand
would be pretty funny if the devices would fail