So, what is my mistake? When and where existed "Ukraine" before 1991?

Even if your premise was true, which it isn't, do we justify the imperialist invasion of a sovereign state because it was founded after some threshold year?

Countries are not a fact of nature. They change, come and go for a large number of reasons. The Ukrainian people are constituted in a country and have repeatedly expressed they don't wish to be part of Russia. If you want to force them to be at gunpoint, at least be open about it instead of hiding behind a historical pseudo-argument.

1918? Most of the territory of modern Ukraine. Before it was conquered by Bolsheviks.

Also Russia as an independent state ceased to exist in 1922. So it’s hardly that different. Both Russia and Ukraine only became independent countries in 1991 when they decided they to leave the USSR.

Hacker News should just ban your account. After all you didn't have it before 2014, so I guess the natural state of the site is not having you around.