Throughout the Cold War, the U. S. regarded NATO as one of the means to contain the Soviet Union in Europe. The formation of the Atlantic Alliance was a Western response to the Soviet threat, which was fully consistent with a “defensive” realist argument. Even after the dissolution of the Soviet Union, however, NATO not only survived the end of the Cold War but also became enlarged and globalized; it conducted crisis response operations in the Balkans and Afghanistan, and trained Iraqi military, with its new members of the former Warsaw Pact. The dynamic transformation of NATO constitutes a challenge to a traditional understanding of alliance.
This article is an attempt to examine how and why NATO has transformed itself into a global “expeditionary force.” The key to unlock the puzzle is alliance's Strategic Concepts (1991/99) and Comprehensive Political Guidance (2006). These strategy documents, decided by consensus of all members, provide a common denominator in alliance management. It contains three important elements. First, alliance's role and mission was redefined. After the Soviet threat vanished, “out-of-area” operations in Bosnia and Kosovo gave NATO a raison d'être. In the wake of 9/11 terrorist attacks, NATO invoked its collective defense clause, which became a prelude to a historic decision to support the Global War on Terror. Second, alliance's capability was redefined. Even somewhat reluctantly, NATO decided to support the U. S. -led coalitions on a case-by-case basis. This new orientation made it necessary to close the “capabilities gap” between the U. S. and Europe. NATO Response Force, proposed by U. S. Defense Secretary Donald Rumsfeld, became a stimulus to accelerate the transformation of European forces. Third, a new form of burden-sharing was explored. In the Cold War era, transatlantic “division of labor” was rather simple; while European efforts focused on conventional buildup for the defense of the continent, Americans continued to offer extended deterrence to European allies. Today, efforts on both sides of the Atlantic became more complementary and even mutually enhancing. American “exit strategy” in Bosnia was made possible by handing over command authority from NATO-led to an EU-led force. In Afghanistan, NATO now assumes overall responsibility inclusive of counterinsurgency operations.
Thus NATO forces have become more agile and deployable in global arena, as stipulated in the strategy documents. NATO is shifting its emphasis from defense of members' territory to defense of common interests worldwide, which corresponds with a prediction of “offensive” realism.

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