Why No No Fly Zone?

 I made this a few days ago and it appears to be still required

Why?

Because a retired general in Canada (as well as some elsewhere) have been calling for NATO to enforce a No Fly Zone over Ukraine.  People seem not to be too concerned about the risk of, well, global thermonuclear war.  So, let's play it out, shall we, with a general X and a Minister of National Defence A:

MND A: So, what do you recommend for us to do?
General X: We should declare a No Fly Zone over Ukraine so that the Russians can't bomb civilian targets.
MND A: What about missiles and artillery?
General X: Oh, the Russians have plenty of those and use them to devastate Ukraine.
MND A: So, why enforce a No Fly Zone?
General X: It is the least we can do, we need to do MOAR!
MND A: Ok, so if we do the NFZ, that means we fly over Ukrainian air space and shoot down Russian planes and helicopters and drones, right?
General X: Well, sure, but we would also want to shoot them down while we are flying in allied airspace to take advantage of how far our weapons can fly and reduce the risks to our pilots from their fighter aircraft.
MND A: Oh, so we would be attacking from allied airspace, but keeping our attacks focused only on planes in Ukrainian airspace.
General X: oh, and anti-aircraft batteries.
MND A: In Ukraine?
General X: well, the Russians have built up anti-aircraft systems nearby--on Russian territory including Kaliningrad. 
MND A:  So, we'd leave those alone, right?  We wouldn't want to attack anything in Russian territory, right?
General X: We'd have to take them out.  To enforce a No Fly Zone, the first step is to take out the anti-aircraft systems of the adversary so that our planes could fly safely.
MND A: So, if we attack Russian weapon systems in Russian territory, what would stop them from doing the same--attacking our weapon systems in allied territory?
General X:  Well, if they did that, they would be escalating and risking a war with NATO as it would potentially lead to the invoking of Article V.
MND A: Let me get this straight, we would attack Russian units in Ukraine and probably Russia, but we would expect them to be restrained?
General X: Sure, because they wouldn't want to start World War III.
MND A: Wouldn't we be starting with our attacks on Russian military units, something that everyone has tried to avoid for about 70 years?
General X: I don't think it would raise the risk of nuclear war by that much.
MND A:  So, it would raise the risk of nuclear war.
General X: Sure, but only by ... say ... 10%. 
MND A: So we have gone from nuclear war being improbable to being possible but not likely?
General X: Well, yeah, but, see, we would be doing something more than what we are doing now.
MND A: And we would only be risking a smidge of nuclear war... General, you are fired.

End scene

 To be fair, Hillier is retired, and retired generals can spout reckless stuff all they want.  The nuclear era is a dangerous place, and one of the rules to avoid nuclear war has long been not to shoot at countries that have nuclear weapons.  Sure, we had some pilots from various opponents engage each other over the skies of Korea and Vietnam, but there was heaps of plausible deniability and no risk of allied/enemy territory being hit.  In this case, the NFZ would be most public--no deniability--and NATO/Russian territory would be implicated.