Over the past several months, NATO has begun fundamentally changing how it provides security to its newest member states.

The alliance’s new enhanced forward presence initiative represents a seismic shift not simply because it is a precedent. For the first time, NATO is basing combat forces on a routine basis within its central and eastern European member states.

It also represents a return to territorial defense, something many had considered a relic of the Cold War in the context of Europe.

For most of the last quarter century, NATO has focused not on collective defense but rather on crisis management and cooperative security beyond member states’ territory, first in the Western Balkans, then in Afghanistan, and, to a lesser extent, in Iraq.

NATO’s embrace of crisis management, cooperative security, and expeditionary operations fundamentally influenced alliance and member state force structure, capabilities and acquisition programs, manpower, training, infrastructure, and defense policy.

However, Russia’s illegal annexation of Crimea in 2014 and its invasion of Ukraine have upended assumptions about the security environment in Europe. As a result, NATO is once again emphasizing the importance of collective defense in its strategy, operations, structure, and training. Despite the alliance’s official position that its other two missions—crisis management and cooperative security—remain central, it has become quite clear that collective defense has returned to the fore as first among equals.

Even as the alliance reembraces its commitment to territorial, collective defense, there are large hurdles standing in its way.

The most significant challenges facing the alliance as it attempts to rebalance toward collective defense can be broken down into roughly three categories: the goals of collective defense, the fiscal means available to member states and the alliance, and the ways chosen to achieve the ends.

With regard to the ends, no single security challenge threatens allied collective defense like Russia. It alone has the power to destroy Europe. Unfortunately, NATO’s long-term strategy toward Moscow appears in disarray. For twenty-five years, the alliance has attempted to treat Russia as a partner, as its member states navigated a complicated post-Cold War security environment in Europe. The alliance, as well as its leading member states, has pursued this strategy with Moscow despite uneven results at best and obvious indications that Russia has been largely uninterested in the kind of partnership that NATO favors. The alliance must now determine whether Russia is an adversary, a partner, both, or neither.

With regard to means, defense spending among NATO allies in Europe is, at long last, on an upward trend. The vast majority of European NATO member states still do not reach the alliance’s target of spending the equivalent of 2 percent of gross domestic product on defense, or investing 20 percent of all defense spending in acquisition. But the new trend is viewed as very good news in Washington as well as within NATO headquarters in Brussels. Inequitable burden sharing is not simply a question of fairness—rather, it has real implications for operations (hence strategy and policy), and ultimately for whether the alliance is merely a paper tiger.

Despite this good news, it’s still not entirely clear what explains this shift in allied defense spending and hence burden sharing. Is it simply a response to Russia’s actions in Ukraine, or are other dynamics at work, such as the threat posed by the Islamic State, the challenge of mass migration, or the need to spur domestic industry?

Depending on the causal variables, policymakers in allied capitals will need to draw the right lessons in order to build upon the trend and ensure that NATO can successfully retool itself for collective defense.

Finally, and with regard to the ways, the alliance and its member states—especially those of European NATO—face significant capability and capacity shortcomings when it comes to their military forces.

It’s no secret that defense austerity has compelled European militaries to reduce equipment holdings, especially that which are most relevant for territorial defense. As as a result, the gap between U.S. and European military capabilities has grown.

Tanks, armored personnel carriers, ground-based air defense, and other critical capabilities have either been downsized or eliminated from inventories, in some cases dramatically affecting the ability of individual European NATO members to field capable, nearly full-spectrum national forces.

Meanwhile, cuts in personnel as a result of defense downsizing as well as military professionalization have meant that there are significantly fewer corps, divisions, and brigades across the alliance today than there were just a few years ago. In place of these larger formations, most European member states today field only smaller formations such as regiments, battalions, and companies.

Replacing worn-out equipment and filling out new or partially full units with adequate personnel are necessary priorities for European defense spending. Moreover, governments of European NATO member states are strongly motivated by domestic politics to favor acquisition of job-creating military hardware, or spending money on personnel salaries and benefits.

But what are the implications of overemphasizing acquisition and personnel, perhaps at the expense of training readiness? The alliance faces the risk that its member states’ military forces may be out of balance, resulting in a lack of readiness, despite NATO’s focus on this issue in recent years.

The difficulties facing the alliance in adapting its strategy, its resources, and its forces for territorial defense in the twenty-first century—while nonetheless remaining committed to crisis management and cooperative security—are daunting, but not insurmountable.

This means NATO recognizing it needs a realistic, competitive approach toward Russia. It needs to reinvigorate its resource pool through continued emphasis on shared threat perceptions as well as maintain a balance between equipment, manpower, and training. The alliance can then succeed in overcoming the most significant challenges on the path back to collective defense.

Dr. John R. Deni is a research professor at the Strategic Studies Institute of the U.S. Army War College and an adjunct professor at the American University’s School of International Service. This piece is based on his newly published book, NATO and Article 5. The views expressed are those of the author and do not necessarily reflect the official policies or positions of the U.S. Department of the Army, the U.S. Department of Defense, or the U.S. government.