Great question.
Let’s put on our skeptics’ hats and be Chamberlain defenders for a second.
Sure — Hitler wrote about conquering the East (maybe “raved” is a better word, since his writings and his speeches were pretty paranoid/delusional). But Germany had never in its entire history attempted to lay claim to that much of the East. Hitler’s vision seemed grandiose and frankly unfeasible. As a matter of fact, it was unfeasible, since Hitler failed. But not without causing millions upon millions of deaths.
I don’t actually know if Chamberlain was just plain ignorant of what Hitler had said/wrote, necessarily. I think it it more probable Chamberlain found it unbelievable: The ravings of a demagogue puffing up his own chest without any real intention of carrying it out.
History, it turns out, proved Chamberlain devastatingly wrong. But, without the benefits of future knowledge, there were rational reasons that supported his efforts at diplomacy.
Now! Let’s turn to Putin!
Putin has explicitly stated his intention to “demilitarize” and lay claim to all of Ukraine. He doesn’t think Ukraine is a real country, and using the megaphone of the state, he’s primed his entire country to believe the same thing. The fact that millions of families have lived for decades on both sides of the border is immaterial. Those family ties are fraying, as Ukrainian families find themselves unable to reach Russian relatives in a literal or a spiritual sense. A sad testament to the power of war propaganda to overcome longstanding ties of blood and kinship.
So, Putin is actively laying the groundwork for the dissolution of the Ukrainian state and seeking the “re-education” of 44 million Ukrainians into Russians. To the extent he is able, he will persecute and destroy the Ukrainian language, Ukrainian monuments, Ukrainian culture, Ukrainian history. He’s already doing this in the areas he controls. All of this, in a word, is genocide.
Ukraine is the largest, most populous country in Eastern Europe. If he’s successful here (in serious doubt, for many reasons, but let’s assume he is), are we to really believe he won’t attempt the same in Moldova? The Baltic States? Romania? Poland?
Russia (as the USSR, and prior, as the Russian Empire) controlled many or all of these regions. Within current lifetimes, even. So unlike Hitler, these ambitions have historical precedent.
Putin’s threats of conquest are extremely credible, as the people of Eastern Europe know best.