Abstract
The article analyses the process of the NATO enlargement after 1990. It starts by a detailed analysis of the secret negotiations which have been started just after the end of the Cold War. In the light of the institutional liberalism, the NATO enlargement is a positive process which satisfied especially new member states. But in the light of the American neorealism, this process resulted into profound changes in the balance of the security threats and into a large militarisation and tension at the new Eastern frontier of NATO in a direct neighbourhood with the Russia. New military units with the modern arms systems are deployed over there and we are witnessing a growing number of dangerous military incidents. As a result, the contemporary situation needs new political negotiations between two competitors and a shift from the contemporary negative Peace towards the positive Peace.