Nukes

 I am confident but not certain that Putin will not use nuclear weapons in Ukraine. My confidence can be largely summarized in this piece by Timothy Snyder. "War is politics by other means," means that war must have a political goal. As Snyder points out, nuclear powers have fought and lost wars since 1950 without resorting the nuclear weapons. Soviet defeat in Afghanistan arguably led to the collapse of the Soviet Union, yet they still did not use nukes. 

Ukraine will not be invading Russia. They might launch raids at supply depots and communications hubs, but Ukrainian troops will not be invading and occupying Russia territory. Therefore, the fundamental purpose of nuclear weapons - deterrence against the destruction of the state - will not be engaged. 

However, Putin's decision making has been flawed, and I can't say with complete certainty that he won't make a catastrophic decision. It just looks really, really unlikely.