They are not stupid enough to nuke us.
Are they smart enough to sink an oil tanker in the Straits of Hormuz thereby throwing the economies of the western nations into depression? Iran, a Russian client state has that capability and enough insanity to do that for sure. What would we do when a deadly strain of small pox kills half of Mexico City and sends 20 million terrified Mexicans straight at our border? Going to shoot them all to keep them from crossing the border? Think that might put a crimp in our economy and divide us ethnically like nothing we've ever experienced before? Biological weapons don't leave radiologic finger prints - who are you going to retaliate against when the wheat rust the Soviet Union developed in the '80s wipes out our entire wheat crop? I could go on for hours positing potential attacks Russia is capable of carrying out without even scratching the surface of what they are capable of. This is no where near as simple as our military will crush them in a heart beat. Wars are not only fought on conventional military battlefields.
Sun Tzu - The Art of War