Russian opposition to NATO enlargement

Russian opposition to NATO enlargement relates to the aftermath of the Revolutions of 1989, when the fall of Soviet-allied communist states to opposition parties brought the Soviet spheres of influence into end. Russian authorities claim that agreement on non-enlargement of NATO to Eastern Europe took place orally and the alliance violated it with its enlargement, while the leaders of the alliance claim that no such promise was made and that such a decision could only be made in writing.

Source: Wikipedia — Russian opposition to NATO enlargement (CC BY-SA 4.0)