Looks like the Russians are in their final preparation stage before invasion. They are moving fighter jets into Belarus and other areas bordering Ukraine, their various fleets are setting sail for a global "exercise" and air defence systems are now in place. The ground is also freezing again, so the Russian tanks can move.
On the issue of nuclear weapons, what is different between now and the Cold War is that Putin has said repeatedly that he thinks a "limited" nuclear strike on a opponent is perfectly feasible. He obviously doesn't believe in the Cold War concept of M.A.D. - Mutually Assured Destruction.
Looks like he's timing things for the start of the Winter Olympics, perhaps sending a litlle message to the Cinese too...??