That's simple whataboutism. What's your point?

Respectfully disagree. Whataboutism is when you try to relativize something by citing an equally bad example from some other side. I'm explicitly not relativizing anything. A whataboutist comment would be "the US does the same [so what Russia is doing is fine", which is the opposite of what I implied: "the US does the same [equally bad thing, we should condemn both]".

The war is an unjustified aggression. It is great that Ukraine has been getting international sympathy and support. It would be great if we all had a similar response in the many other comparable cases that unfortunately happened in the past.

It's an equal deflection from the main point that adds very little to the discussion. You could have equally said "The war is an unjustified aggression BUT the USA has been doing that for a long time too so..." and the message would have been the same. You just phrased it differently.

I feel like nobody can read the actual definition of this term, and how and why it is used in a conversation.

Sure you can.

https://en.wikipedia.org/wiki/Whataboutism

https://www.merriam-webster.com/words-at-play/whataboutism-o...

The reason you have to write that disclaimer is that you're by definition being a "useful idiot" by repeating the favorite argument of the Russian regime: whataboutism/tu quoque. What the US has done in the past or is doing right now is completely irrelevant when judging Russia's actions in Ukraine.