Was NATO expansion a big mistake?

Nope. Why would it be? If anything, the current Russian invasion is just going to push other countries to join. Georgia and Moldova just applied. And popular support for it in Sweden and Finland is at an all time high.

NATO expansion is just Western imperialism expansion, which is totally acceptable and justified if you live in a western country or any of its colonies.

This war is just Russian imperialism expansion, which is totally acceptable and justified if you live there, but not acceptable nor can be justified from the point of view of the western imperialism, not because western countries think its morally wrong, but it clashes with its own interests.

NATO was originally created to curb Soviet Union. Then Soviet Union went away, and there was a period of time where it seemed that NATO is no longer needed, because Russia no longer seemed like an offensive and expansionist nation.

That theory was obviously false, and we should have seen in 2014 that it was. Perhaps many people actually did.

If there's a mistake in here, it is that not more countries are in NATO yet.

For most of the world? Yes.

For the people who own the United States? No.

Yes. NATO started as an alliance with an existential purpose, meaning that each and every member felt that an attack on any of them would threaten their very existence. Its expansion after the fall of the Soviet Union was paradoxical, because the threat was vastly reduced. And now we have a situation where a Russian attack on, say, North Macedonia would immediately trigger World War. I don't see how that is beneficial to anyone except of course people from countries such as these. But it's unclear that we should risk world stability for the sake of the safety of ex-Soviet republics.

No. This had to come to a head eventually. Russia has to end its dependence on the tsarist power structure and join the modern cooperative world. We don't live in the middle ages anymore. However, Bush's hard push was definitely a foolish move that made things worse than they could have been.

What's happening now is (unfortunately) probably the least bloody way to topple Russia's (hopefully?) last tsar. But even then it's hard to say if this is really the end of Ivan the Terrible's legacy... We also thought that Gorbachev and Yeltsin marked the end. Old habits die hard.

The only mistake was that Russia, Ukraine and so on weren't strongly invited to join NATO after the fall of USSR. I hope it'll be fixed this time around.