During the negotiations over the reunification of Germany in 1990, US Secretary of State James Baker made a verbal promise to Soviet Union Foreign Minister Alexander Bessmertnykh that NATO would not expand eastward if Russia accepted the reunification of Germany.

The promise was made the debate is over its enforceability. Of course this promise meant nothing same as the Budapest Memorandum on Security Assurances also meant nothing.

So basicly putin is a hypocritical bastard by claiming the whole nato wont expand thing was a broken promise while simultaneously breaking the Budapest Memorandum.

EDIT: found the source.