Which commonly proposed actions by NATO or EU members would have a high likelihood of causing a war with Russia if neither side backs down?
I realize that this is a rather broad question, but I find myself writing the same answer to different questions, often several times a day. "Yes, the West could do that. And then a war between the West and Russia becomes rather likely."
a b c d e f g
For NATO or EU actions, presume that they are going to carry through with their promises, i.e. NATO membership for Ukraine would trigger Article 5, EU membership would trigger Article 42.7, and a no-fly zone would not just be declared but actually enforced. For Russian reactions, one would probably have to go with their declared policy, perhaps with a grain of common sense added to the mix.
The wisdom of triggering WWIII over Ukraine is out of scope. We might have a Munich moment or a Cuban Missile moment with the Russian invasion, only hindsight will be able to tell.