THE NORTH Atlantic Treaty Organization is one of the most successful alliances of all time, but after the cold war and the successful completion of its mission, NATO suffered an identity crisis. It now has three main functions and self-images that compete with each other. The first persona is the enforcer, the pacifier of conflicts beyond the region's borders; the second is the gentlemen's club for liberal and liberalizing countries of the West; and the third is the residual function of an anti-Russia alliance.

One must wonder why, with the end of the cold war, NATO did not dissolve. How do we explain the organization's transformation and vitality at the end of the twentieth century? NATO did not retire after victory because it was not just any old alliance. Rather, it had become a genuine institution, complete with transnational, integrated command structures, a permanent bureaucracy, buildings, regular meetings and ceremonies, its own logo, website and so on. Institutions take on lives of their own; they have a self-preservation instinct and successful ones especially want to keep validating their importance. This is the "March of Dimes" explanation for NATO's persistence after the collapse of the Soviet threat. And though NATO's attempt to survive might be expected, its evolution over the past twenty years is in many ways a paradox. In one of its personalities it became a more muscular and combative military alliance just when it least needed to be: after accomplishing its strategic purpose. In another of its personalities, however, it stopped taking its core military mission seriously. The tensions among these three personas may be less dramatic than in the famous film The Three Faces of Eve; with luck the West might easily live with them indefinitely.

But what if luck runs out? These identities are a potentially corrosive mix, particularly as they relate to Russia. And a hint of the possible risks to come was seen last August when Russia and Georgia came to blows over two separatist regions, South Ossetia and Abkhazia, raising worries in the West about being dragged into the imbroglio. Despite such a wake-up call, however, policy makers have avoided confronting NATO's identity issues; many do not even recognize them or see them as troublesome. Since the organization is still dominated by the United States, tensions among these personalities are, more than anything, Washington's problems. And if the problems come to divide European members from Washington, the epochal success of the organization will give way to overreach and decline.

THOUGH NATO was created to prevent World War III, it did this for forty years by preparing to defend and retaliate if the Soviet Union attacked any of its members, and it went on to win the cold war without once engaging in combat as an organization. However, within a dozen years afterward it had gone from having a purely defensive posture to adopting in effect, though not in principle, an offensive one, engaging in two hot wars, over Kosovo and in Afghanistan-or three, if the brief 1995 Allied Force campaign against the Bosnian Serbs counts. NATO's out-of-area enforcer identity was evolving. All of these combat actions have been against adversaries never envisioned at the time the organization formed and matured. The alliance's two actions in the Balkans were attacks initiated by NATO against countries that had not attacked any alliance members. In humanitarian terms, the war for Kosovo may have been defensive, but in terms of interstate relations it was anything but. In any case, the alliance's new slogan became, in the famous words of Senator Richard Lugar, "Out of area or out of business."

And here we see the first signs of trouble. Staying in business by getting into the Balkans was a mixed experience. NATO prevailed in Bosnia and Kosovo, but without a consensus that the price paid was worthwhile and without assurance to date that either of those places will not erupt again. NATO keeps biting off more than it can chew. The mission in Afghanistan, of course, is the biggest open question.

To be sure, invading Afghanistan was a defensive response to al-Qaeda's assault on the United States. After September 11, the European members gallantly rallied to their stricken ally and offered to participate in action against the Taliban regime. Ironically, the offer was not appreciated at the time because Washington did not want the allies getting in the way of the more proficient American military machine. But as the initial victory of 2001-02 has crumbled in recent years, the United States is asking for more help, not less, from the Europeans and Canadians, whose forces have outnumbered American troops in the country and borne a fair share of the burden of combat. More recently the European allies balked at American demands to do more. The alliance is looking less than united.

So, though this might be a clearer replica of the type of mission NATO was designed for-to protect an ally once it had been attacked-the unfolding of the dirty little war has exposed too the practical complications and limits of the most integrated alliance in history. Military operations in Afghanistan have been divided between two chains of command, one under NATO and one an independent American channel. Domestic political controversies within Europe over participation in the mission inhibit the types of operations certain countries' contingents perform and raise questions about how long all will remain engaged. If this were fully a NATO war, organizational logic would put the Supreme Allied Commander Europe (SACEUR) in charge, but practical strategic logic has made the U.S. Central Command (CENTCOM), which encompasses Pakistan, the main overseer. This sort of managerial awkwardness can be handled, but if the strains of the conflict magnify disagreements among NATO capitals about how to fight the war, the symbolic unity of the alliance will fade. As the Obama administration reboots strategy for a stronger effort in Afghanistan, getting on the same page with the allies could prove to be less than a seamless exercise. This crisis of military identity is the most immediate issue on which NATO faces real problems to solve rather than triumphs to celebrate.

BUT THE organization has long indulged in triumphalism, most readily in its expansion post-1991. Even while concentrating on military defense during the cold war, NATO always had a secondary purpose of serving as a diplomatic vehicle for transatlantic political unity. After finally disposing of the Soviet Union, however, the political persona took over. In the years after the Berlin wall fell, NATO's main function became to serve as a political club, to celebrate and consolidate the democratization of the Continent by bringing the liberated countries of the Warsaw Pact into the Western fold. The organization not only declined to retire, it did not even stand down after victory. Instead it nearly doubled in size and rolled itself right up to Russia's door.

Though this political-unification function might seem to more naturally fall to the European Union (whose inclusion of countries from the old Soviet empire proceeded much less expeditiously) it was instead performed by NATO because the United States was simply not a member of the other continental club and so relied on the Atlantic alliance to keep Washington in the driver's seat. Through this mechanism, the United States extended its reach further into Europe. This was especially true regarding the new entrants to the organization in the East, what former-Secretary of Defense Donald Rumsfeld called "the New Europe"-from Poland and the Czech Republic to Bulgaria and Slovakia.

This decision was made even easier because for the Europeans the EU was not to be the vehicle for political consolidation because the EU was serious business. To most politicians after the cold war, willingness to wage hot war to defend new members-the core of the North Atlantic Treaty, embodied in its Article V-was an irrelevant abstraction because the sort of war that NATO had been developed to handle had become utterly implausible. EU membership, on the other hand, involved money! For a long time it was more politically difficult to let Poland sell tomatoes in France than to give Warsaw a pledge to fight and die to save it. And here we begin to see the less immediate, but perhaps deeper, problems of the alliance's identity crisis.

The Bush administration and most of the foreign-policy establishment in both American political parties wanted to continue expansion through Ukraine and Georgia. Eventually the states were to follow the Baltics into NATO, even though Russian support for secession of Georgia's breakaway provinces, Abkhazia and South Ossetia, was well established long before the August engagements. What were they thinking? That the breakaway regions would voluntarily reintegrate with Georgia before the country was admitted to NATO? That Georgia would regain the territories by force without Russian intervention? That Georgia would be admitted without the two separatist regions and NATO would add a codicil to admission that its defense guarantee excluded them? Impossible.

Yet many observers still endorse Georgia's eventual admission as well as Ukraine's. And Ukraine, which involves much-bigger stakes, has potentially explosive internal cleavages as well: a Europe-oriented population in the western part of the country and a large Russian minority in the east. Somehow the idea that a country's membership in NATO obliges the United States to go to war with Russia should issues of Article V arise escapes much of the American political elite. Indeed, the alliance may have become a pacifying gentlemen's club, but its anti-Russia stance is where the only big danger lies. And so the idea of an expanding liberalizing elite starts to look less genteel and more aggressive.