Stop it with the no fly zone stuff. Nuclear powers do not engage directly, full stop. The US/EU will NOT do this, because the potential consequences are too severe.
-W
We have in the past, in fact I'd estimate over the 70 years we've directly more than 100 of each other citizens, and indirectly 10,000.
Let's review
in 1960 Col Powers was shot down over the Soviet Union, according to an account I read every single pilot in his U2 recon squadron was also attacked and two were hit, but landed in Turkey.
In the Vietnam war Russian pilots flew MIGs that shot down US, Russian technicians also manned North Vietnam SAM (Surface to Air), which destroyed 900 US Aircraft.
US attacked the SAM sites killing dozens of Russians
In 1972, the US mined the harbors of Hanoi, Haiphong will Russian ships were in port
The USSR was the primary weapons supplier to North Vietnam a war that killed 58,000 Americans
From 1979-1989 the US was the primary supplier to the Mujahadeen in Afghanistan, the USSR lost nearly 15,000 in that war
In 1983 Russia shot down a US spy plane, which turned out to be a Korean Airliner, killing 62 US citizens.
From 1991 to 2002 the US maintained No fly zone over most of Iraq. Saddam Hussein was a big customer of the USSR Air Defense, which were routinely manned by Soviet technicians. The SAM sites were also routinely destroyed by US planes, killing the Russians.
In 2018, Iranian militia and Russian contractors, attack US special forces and Syrian forces. Account very widely but most say around 25 Russians were killed.
This list ignores, the countless, SAM launches, missile locks, a couple of air-air collisions, and some near-collisions between US and Russian ships.
It also ignores special operations, where undoubtedly both sides killed each soldiers. It is also ignore spy operations, assassinations, and quite successful interference in the 2016 election.
In short the cold war was often pretty hot.
The assumption, that a No-Fly Zone in Ukraine would lead to an all-out air war, which in would turn lead to WWIII is faulty. We've spent 70 years figuring out how to avoid using nuclear weapons in some very tense situations. If we didn't go to war over ten of thousands of troops being sent home in body bags in Vietnam and Afghanistan, would Putin really start WWIII over even a 100 planes being shutdown? Would his military let him, or would they respond like Gen Milley did with Trump and work around a crazy man? These are all unknowable questions.
What we do know is this
1. Not having air superiority will make it much harder for Russia to take over Ukraine
2. Sanctions almost never work, and when they do like in South Africa they take decades. Ukrainians don't have decades they have days perhaps weeks before the Russians start blasting them with artillery, airstrikes and helicopter gunship
3. The chess proverb the threat is sometimes stronger than the execution applies to warfare. NATO doesn't necessarily need to implement a No-Fly zone, merely the credible threat may be sufficient to bring Putin to the negotiating table.
4. It is a very dangerous precedent to establish that as long as you have enough nuclear weapons, you can bully, and attack your neighbors with no fear of being attacked by conventional means.