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.
At some point I saved this twitter thread:
https://nitter.it/RnaudBertrand/status/1498491107902062592#m
It contains an opinion of Kennan and other US and foreign diplomats expressing this opinion.
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.
I think this is what you're looking for:
But most realists opposed expansion, in the belief that a declining great power with an aging population and a one-dimensional economy did not in fact need to be contained. And they feared that enlargement would only give Moscow an incentive to cause trouble in eastern Europe. The U.S. diplomat George Kennan articulated this perspective in a 1998 interview, shortly after the U.S. Senate approved the first round of NATO expansion. “I think the Russians will gradually react quite adversely and it will affect their policies,” he said. “I think it is a tragic mistake. There was no reason for this whatsoever. No one was threatening anyone else.”
(PDF) Why the Ukraine Crisis Is the West's fault by John Mearsheimer, originally published in Foreign Affairs in 2014
Love how there's one of these clips for half of the things he's fucked up lol
