It's something like 4000 miles from there to Moscow, and 2000 miles to California. In both cases most of the trip would be across wilderness terrain with no major roads or infrastructure to speak of. In the age of nuclear weapons, the war would be over long before an army crossing the Bering Strait reached anything remotely important. Even if nuclear war did not break out, a large ground force making such a long and difficult journey would have little hope of survival. They would be hit by air strikes for weeks or months, and then would face a prepared defense if they ever made it to their goal.