Russia annex 4 Ukrainian regions
Ukraine attacks those regions in an attempt to retake them back using Western supplied arms
Eventually Ukraine counteroffensive seems promising,
Russia uses tactical nukes upfront Ukraine's frontline,
The nuclear radiation fallout reaches Poland, Lithuania
Poland, Lithuania triggers NATO article 5, "we're under attack "
NATO bases on all Baltic States alerted,
Flocks of F35, F22, F16, F15, fleets has just owned the eastern European sky in just an hour. No fly zone created around all of Ukraine.
Russia orders another nuke launched towards Ukraine, in an attempt to protect annexed territories.
There are some disagreement between Kremlin generals, some are executed for denying orders.
Before it launched, NATO retaliate by completely dismantling any ability of Russia second strike: nuke silos, subs, radar, blacksea fleet, personnel.
Russia in panic mode, attempts to launch of what is remained in its arsenal,
NATO all in.
Moscow and st Petersburg falls by Monday.