Watch How NATO Leaders Respond to Brexit at The 'Most Important' Summit in 25 Years

When NATO leaders meet in Warsaw later this week, it will be the most important such gathering since the end of the Cold War a quarter century ago.

Two years after annexing Crimea and invading Ukraine, Russia shows no sign of stepping back from its efforts to challenge NATO’s cohesion or threaten its territory by way of a massive military build-up on the alliance’s borders. Meanwhile, Europe’s south is the source of increasing threats to security — ranging from ISIS-trained and inspired terrorists to large-scale migration from the conflict-ridden and economically destitute areas of South Asia, the Middle East, and sub-Saharan Africa. To top it all off, the British vote to leave the European Union poses the gravest challenge to the European project and European unity in more than half a century.

The task of NATO leaders is to put in place policies and processes that respond to this triple challenge to Europe’s security in ways that are clearly understood by the alliance’s adversaries and supported by increasingly restive publics on both sides of the Atlantic. It is a tall task, indeed.

The alliance is best placed to respond to Russia’s continuing threat to European security—a threat that seems to be unrelenting in its focus and effort, as underscored by the 2,000 exercises and wargames Russian military forces are planning to conduct between June and October. In response, NATO leaders this week are expected to announce the forward deployment of four combat battalions (about 1,000 troops each) to the three Baltic States and Poland. These forces, which will deploy on a rotating basis and be led by Britain, Canada, Germany, and the United States respectively, will provide a demonstrable commitment of NATO to the defense of alliance territory. Their persistent presence in Eastern Europe also provides a clear warning to Moscow not to infringe on NATO’s territorial integrity or its members’ political independence.

While taking steps to bolster deterrence, including by proceeding with the deployment of missile defense systems in Poland, NATO leaders will also offer an opening to dialogue with Russia. NATO’s Secretary General has already announced that he will convene an ambassadorial meeting of the NATO-Russia Council following the Warsaw Summit, and higher-level dialogues could also be in the offing. There is no contradiction between deterrence and dialogue; in fact, to the extent the latter can reduce misunderstanding and provide for better communications, deterrence and security will be enhanced.

Addressing the threat from Russia enjoys widespread support throughout the Alliance, even if those most exposed to the Russian military challenge are understandably the most insistent on proactive measures of deterrence and reassurance. The same is much less true when it comes to the security challenges emanating from the south. To date, NATO has taken a distinctly secondary role in addressing the threats of terrorism and preventing large-scale migration across the Mediterranean and Aegean Seas. But that could well change, as the European Union, newly embroiled in its own crisis, may prove unable to address these challenges on its own.

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Related: Defense One's complete coverage of NATO

There is, indeed, much that NATO could do to address the threats from the south. Its naval vessels could augment those of the EU in patrolling the seas and go after the organized criminals who have exploited the misery of desperate people by smuggling them for great sums of money in rickety boats across the waters. NATO Airborne early warning and surveillance assets, including NATO-owned and operated AWACS airplanes, could enhance the effectiveness of that effort.

These surveillance capabilities could also contribute to the fight against terrorism, including by sharing critical information with the US-led counter-ISIS campaign in Syria and Iraq. NATO can also enhance its training of security forces in Iraq, extending the effort to Libyan security forces, and assist in bolstering the Syrian opposition. Finally, the Alliance could use the intelligence backbones of its civilian and military command chains to share critical information about terrorist activities and actions. Ultimately, of course, NATO could take overall command and control of the counter-ISIS mission, much as it did in Afghanistan.

Alliance leaders are unlikely to agree to all of these steps. But the idea that NATO cannot do more to address these types of security challenges, which many of our citizens believe to be at least as large as the threat emanating from the East, is clearly mistaken. To be relevant, NATO must demonstrate it has the capability and will to respond to the challenges and threats that are most immediate in the eyes of the people NATO exists to protect.

NATO’s success in addressing the security threats of today and tomorrow will depend in large measure on how it responds to the unexpected British decision to leave the European Union. For all its quirks and faults, the EU stands at the core of the European project, which transformed a continent that was at war for centuries into the largest and most prosperous zone of peace and prosperity in history. For more than 65 years, NATO provided the security foundation upon which the European Union was built. Brexit puts that entire project in doubt.

Although formally nothing changes in terms of the UK’s membership in the Atlantic alliance, the chaotic political situation in London and Brussels that resulted from the Brexit vote will invariably lead to a more inward-looking Britain and EU at a time when the growing threats to European security require a robust and outward-looking response. The unnecessary distraction, which likely will last for years, will clearly have a negative impact on NATO’s ability to act forcefully in meeting the challenges it faces.

That said, the Warsaw Summit provides the occasion for all leaders, on both sides of the Atlantic, to reaffirm the centrality of the European project, which is built on economic openness, political solidarity, and collective defense. When nations turn inward, the security and prosperity of all inevitably suffer. Warsaw is the time to act; to reaffirm our commitment to common security through increased unity and cooperation.

Ivo H. Daalder is president of the Chicago Council on Global Affairs. He was U.S. ambassador to NATO from 2009 to 2013.