Sorry if this is too vague, but I remember hearing that there was some kind of group of diplomats and ghouls back in the 90s or early 2000s that specifically warned against NATO expansion as an unnecessary provocation of Russia. I've found George Kennan's opinion piece, but could have swore there was a letter or statement is some kind from a larger group. All my google results just turn up a million Ukraine war propaganda pieces.
deleted by creator
I agree, and to be honest at least for me the point is not that that nobody listened to them but rather the inverse: since many experts and people in the US political ranks (including the current head of the CIA) were making this claim, it is unlikely that it's simply a US blunder but rather a calculated push from the US to create tension and start a proxy war (of some kind) with Russia. Even though I am not sure the US was really aiming for a full blown war (as it happened), but maybe they would have preferred just an increased involvement of Russia in the Donbas civil war.
deleted by creator