We don't need to go into that long history. Most of these CEE countries have been invaded by Russia in the last 100 years.
> It’s not unexpected that the former Warsaw Pact countries would want to join. It is surprising to see the US being so short-sighted to allow these expansions considering that several of its own diplomats and foreign policy experts were warning against it.
It actually worked out well. Not letting e.g. Baltics into NATO/EU would almost certainly mean they have been already absorbed back into Russia. The rest of CEE would be vassalized.