From an article in Responsible Statecraft by Blaise Malley: Declassified Docs: US Knew Russia Felt 'Snookered' By NATO

'In 1995, then-national security adviser Anthony Lake warned President Bill Clinton that Russian leadership would not accept the expansion of the alliance to the East.

'“Russian opposition to NATO enlargement is unlikely to yield in the near or medium term to some kind of grudging endorsement; Russia’s opposition is deep and profound,” Lake wrote. “For the period ahead, the Russian leadership will do its level best to derail our policy, given its conviction that any eastward expansion of NATO is at root antithetical to Russia’s long-term interests.”

'Two years later, as Washington and Moscow were entering negotiations on the future of NATO-Russia cooperation, State Department official Dennis Ross wrote what the Archive calls an “astute and empathetic analysis” of the Russian position on NATO expansion.

'“To begin with, the Russians for all the reasons you know see NATO expansion through a political, psychological, and historical lens,” Ross wrote in a memo to Strobe Talbott, then the Deputy Secretary of State.

'“First they feel they were snookered at the time of German unification. As you noted with me, [former Secretary of State James] Baker's promises on not extending NATO military presence into what was East Germany were part of a perceived commitment not to expand the Alliance eastward,” the memo continues.”In addition, the 1991 promise to begin to transform NATO from a military alliance into a political alliance was part of the Soviet explanation for accepting a unified Germany in NATO.”

"Because these perceived promises were never made concretely, Ross says, the Russians were “taking the lessons of 1991 and are trying to apply them now in the negotiations on NATO expansion.”

'Despite these roadblocks, Clinton and his Russian counterpart Boris Yeltsin nonetheless reached an agreement on a series of issues at a summit in Helsinki one month later. During a private conversation with Clinton at that summit — which was part of the set of declassified documents — Yeltsin would say that he reached an agreement with NATO not because he wanted to “‘but because it is a forced step.”

'In his exchange with the American president, Yeltsin made one thing apparent. “[NATO] enlargement should also not embrace the former Soviet republics,” he said. “I cannot sign any agreement without such language. Especially Ukraine. If you get them involved, it will create difficulties in our talks with Ukraine on a number of issues.” Clinton did not agree to a “gentlemen’s agreement” to that effect, and the two men eventually moved on.'