Explaining NATO's Resilience: Is International Relations Theory Useful?

Over the past two decades, NATO's membership and mission have expanded. Practitioners and scholars have extensively debated the pros and cons of the changes, but few have sought to explain why the alliance is so resilient. This paper directly asks and answers the question: Why has NATO mission and membership expanded since the end of the Cold War? Three leading approaches of international relations are used to answer this question. The empirical analysis finds that in isolation, none of the theories perform well at predicting or explaining NATO's adaptation to the post-Cold War environment. However, when combined, they do offer a coherent explanation for the alliance's enlargement and the development of out-of-area operations.