Putin annexes (parts of) four Ukraine oblasts, as we always knew he would, I said so months ago. Any attack on them is now an attack on Russia, according to him, even though the Russians are not fully in control of those areas. This gives him the weak excuse to use tactical nukes should the Ukrainian counter-attacks in those areas force his forces back.
The Ukrainian reaction leads them to expedite its inclusion into NATO and possibly the EU. That would bring NATO forces directly into opposition with Russian forces. This would not be a Cold War, it would be a hot one.
If cool heads do not prevail then MAD really could be enacted. The Kherson area south of the Dnipro river, where Ukraine is making advances, could be the first to see tactical nukes.
This is not going well, for either side.