Direct NATO intervention.. They will deploy troops, i mean real one not covert in Ukraine.
Which means total deterioration of diplomatic channels, which also means the nuclear deterrent policy has failed to function as intended and the Russian MOD would now be forced to deploy nukes into battle literally by law, in order to restore faith in their nuclear deterrence policy.
Like I already said it is highly likely the US will put NATO pilots flying F-16s in Ukraine. They might be flying latest F-16s with AESA radar and AIM-120D. With the provision that they can't operate East of the Dnieper so the pilots won't get captured by the Russians.
Putin already signed a decree in 2020 that states two very important and legally binding points.
Russia reserves the right to use nuclear weapons in response to:
1. A nuclear strike on Russia.
2. Conventional aggression that threatens Russia's existence, aka Russia losing the conflict in Ukraine.