The central argument of the study is that the NATO enlargement of 1999 failed to meet its strategic purpose. First, it has not removed the ‘eastern security dilemma’ which underpinned Polish, Czech and Hungarian quest for NATO membership. Second, it did not strengthen NATO political cohesion and military effectiveness. To support the argument the study mobilizes the concept of the ‘security community’ developed by Karl Deutsch in 'Political community and North Atlantic Area'. On the account of Deutsch’s concept the study approaches NATO as a security community. It sees therefore the NATO expansion through the prism of extending three features which characterize a security community: the meaning and nature of ‘trust’; ‘security’, and community’s capabilities (i.e. its ‘power’ and ‘responsiveness’). Because of the nature of these constructs, enlargement policy should not been seen as a one way process where candidates simply adjust to NATO practices and implement alliance’s rules and norms, but as a process of mutual learning and socialisation. For that reason NATO enlargement was a process of rebuilding the community; it challenged the community identity, its security concept, relationships among members, and also community capabilities; ‘power’ and ‘responsiveness’. Against this preposition, the study claims that the policy of NATO enlargement created a strategic ambiguity. First, a ‘trusting-relation’ developed only between candidates and community core of strength, that is the US. Second, rather than forge a new ‘concept of security’ the enlarged NATO accommodated competing visions of the alliance security. Third, new members failed to meet enlargement criteria pertaining to NATO capabilities due to the institutional and economic ineffectiveness and flaws in the NATO conditionality policy. As a result of it, NATO enlargement sharpened and multiplied alliance’s problems, weakened its political cohesion, and thus confronted the new members with politically and militarily challenges they were not prepared for. All this seems to contribute to NATO identity crisis and a growing feeling of insecurity among Central European NATO members.