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In 1949, twelve nations formed a regional alliance in an effort to counter perceived Soviet expansion, creating the North Atlantic Treaty Organization (NATO). Collective defense-assisting member states under attack by an outside country, as articulated in Article Five of the North Atlantic Treaty-has historically been its core function. Following fifty years of dormancy, Article Five was invoked after the September 11, 2001 terrorist attacks on the United States. This Note discusses the expanding role of Article Five, increasing regionalization challenges, and whether the Treaty's obligations compel affirmative actions from its members.