Former U.S. Secretary of Defense Robert Gates meets with NATO Ministers of Defense and of Foreign Affairs in October 2010 at NATO headquarters in Brussels, Belgium. Image: U.S. Department of Defense. The recent support by Afghanistan’s Loya Jirga of an Afghan-U.S. bilateral security agreement marks the end of an era for NATO. While the pending security agreement will help pave the way for a continued NATO presence in Afghanistan until 2024, the alliance’s future involvement in the country will be significantly smaller than over the past decade. As the International Security Assistance Force (ISAF) mission winds down, a bigger debate over what the alliance should focus on in coming years will intensify. This debate is very important not just to the alliance itself, but to transatlantic relations more broadly. Unless NATO can continue to show relevance after Afghanistan, the future of the alliance might well—to paraphrase former Defense Secretary Robert Gates—look dim and dismal.

True enough, Afghanistan has played a key role in shaping transatlantic relations over the past decade. Responding to Afghanistan provided NATO with a justification for its own existence in the new post-9/11 security context by undertaking a complex out-of-area crisis management mission. Though not the first time NATO allies had operated together outside the immediate North Atlantic area—they had previously undertaken stabilization and reconstruction efforts on the Balkans—the Afghanistan mission was the first “deep-out-of-area” mission ever attempted. But Afghanistan also proved more difficult and complex than anything the alliance had previously attempted.

The most extensive and longest lasting mission ever undertaken by NATO, the ISAF mission has absorbed tremendous resources and political attention. Moreover, commensurate with the worsening situation on the ground, public support on both sides of the Atlantic for the mission gradually eroded. The mission’s high human and fiscal costs have also damaged alliance solidarity. NATO allies today are less likely to act in unison. Increasingly, NATO contributions will take place on an “opt-in” basis—as was the case during the 2011 Libya intervention—rather than as a genuinely collective effort, which was the norm up until recently. Emerging out of Afghanistan is therefore a “two-tier alliance,” with a core group of states willing and able to carry out military interventions and with others remaining at the sidelines.

As a result, future NATO missions will be significantly smaller in both scope and scale. Large-scale nation-building—especially when it occurs in faraway places with little direct strategic significance—is definitely a thing of the past. And when NATO does intervene, it will do so using a much smaller footprint. But if this is the case, what should NATO’s post-Afghanistan security role be?

There is a great sense of urgency to the question. The strategic environment in which NATO operates is rapidly changing. The U.S. pivot to Asia and the Eurocrisis, combined with unprecedented security challenges in Europe’s southern neighborhood, all form the backdrop of transatlantic relations in the coming years. Moreover, Washington has repeatedly sent the message to its European allies that it expects them to share a bigger piece of the security pie for the United States to continue investing in the transatlantic relationship.

Herein lies an apparent paradox: in order to keep NATO relevant in the 21st century, the alliance must manage both its traditional territorial defense tasks and take on new ones relevant to global security, all while avoiding costly and long-lasting missions far from the North Atlantic. What does this leave NATO to do?

First of all, NATO must take steps to preserve some of the progress it has made in Afghanistan when it comes to promoting interoperability between European and American forces. It can do so by boosting the number of joint trainings and exercises. This is particularly relevant given the likelihood of withdrawal of some of the American troops from Europe in coming years. Fortunately, this process of promoting interoperability has already started (e.g. the Connected Forces Initiative), but more should be done.

Another obvious opportunity for NATO is in Europe’s wider neighborhood, an area that is of course highly relevant in the post-Arab Spring security environment. Finally, NATO should seek to become a platform for advancing new global partnerships with current partners such as Australia and Japan as well as with regional security organizations such as the Gulf Cooperation Council and the African Union. One regional area where NATO can fill a void is the Arctic, where it could work with other regional players to promote a peaceful development in the region.

The end of the Afghanistan mission marks the beginning of a new era for NATO. Clearly, the alliance will not attempt another Afghan mission anytime soon. Yet, returning to its original purpose of territorial defense is not a viable option either. Instead, NATO must seek to become more dynamic in terms of allies’ commitments and more diverse in terms of missions while keeping focused on new security challenges in order to remain relevant in today’s rapidly changing security environment.