NATO enlargement has been a politically contested subject, since it has been seen as bringing stability as well as instability. Enlargement has been a thorny issue within the Alliance itself, as not all countries have been equally enthusiastic about the decisions to invite new members. Moreover, NATO enlargement has often been a dividing issue domestically when countries have pondered whether joining NATO or supporting NATO enlargement would be a wise policy choice for them. Finally, NATO enlargement has been a hot topic in relation to the Alliance's and its members’ relations with third parties, particularly Russia. While Russia has not been able to prevent NATO from enlarging, as NATO has regarded the decisions concerning military alignment as a right of sovereign states in accordance with the Organization for Security and Co-operation in Europe (OSCE) principles, this has led to questions over what are legitimate rules concerning alliance enlargements within a shared security regime.

At the same time, NATO enlargement has been a puzzle for researchers aiming to explain why and how it has taken place and considering the consequences and normative principles of NATO enlargement. There is little scholarly consensus, but researchers and foreign policy pundits have been as divided in their interpretations and policy recommendations as decision makers and politicians. Scholars of international relations and diplomatic history disagree over the primary motivations and key dynamics of NATO enlargements as well as the effects (see, for example, Ball 1998; McGwire 1998; Kupchan 2000; Rauchhaus 2000).