Workers take measurements of a 3.7 kilometer-long fence at a border crossing between Austria and Slovenia | Joe Klamar/AFP/Getty Opinion Ghost of European re-nationalism An inward-focused, self-absorbed EU is a major threat to the transatlantic alliance.

NATO defense ministers meet this week in Brussels to discuss the two biggest issues testing transatlantic security: a newly aggressive Russia and the terrorist threat posed by a burgeoning Islamic State of Iraq and the Levant.

Left unaddressed, however, is Europe's most immediate and perhaps most significant challenge — the re-nationalization of European politics, which poses an existential threat to the European Union. Ultimately, confronting this immediate crisis will be critical to any success in meeting the challenges posed by Russia and ISIL.

When it comes to Europe's present security concerns — Russia and ISIL — NATO defense ministers have reason for some optimism.

U.S. Defense Secretary Ash Carter will arrive in Brussels on the heels of his announcement that the U.S. is significantly beefing up its military presence and capability in Europe. Washington will quadruple annual spending on European reassurance efforts — including sending more forward-located combat equipment to the European front lines, increased investment in critical infrastructure to enable rapid reinforcement, and additional funds for exercising and training with other NATO forces.

Importantly, the new commitment also includes the additional deployment of a combat brigade into Europe on a rotating basis.

The expanded U.S. efforts come at a time when European governments have finally stopped cutting their defense spending and actually begun to increase their investment.

The new U.K. strategic and defense review included a commitment to keep spending 2 percent of GDP on defense, including by increased procurement of much needed equipment. After years of holding spending steady, Germany announced a modest increase, much of it going to new procurement.

Countries in Eastern Europe are also making major commitments to boost capabilities, with most of the frontline states pledging to achieve the NATO 2 percent level by the end of this decade, if not before.

Coming on top of the reconfiguration of NATO defense plans and the formation of rapidly deployable units agreed to at the Wales Summit in 2014, all these steps are welcome signs of a commitment to defend NATO territory. There is little doubt Moscow understands this commitment and will think twice before openly threatening or attacking any of NATO’s territory.

The challenge posed by ISIL is more immediate and difficult to address. This, too, will be on the defense ministers’ agenda, with Carter urging his colleagues to increase their military commitments — of special forces to conduct counterterrorism operations; trainers for Kurdish and Iraqi troops; combat aircraft to bomb terrorist targets in Syria; NATO surveillance planes to assist the air operations; and funds and weapons to supply the anti-ISIL forces.

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After the Paris attacks, most countries are prepared to do more to defeat Islamic State, though whether the overall commitment will be sufficient to deal it a real setback remains to be seen.

The challenges posed by Russia and ISIL are real and significant, but the good news is that NATO provides an effective forum for major countries to get together, agree on coordinated strategies, and force even the most reluctant countries to make increased commitments. Not so, when it comes to the other major challenge confronting Europe — the re-nationalization of European politics.

Europe today faces an existential crisis. The eurozone crisis has been deferred rather than resolved — and many of the biggest costs have been imposed on just a few countries. Europe dealt with the refugee crisis in 2015 by having a handful of countries, led by Germany and Sweden, and also including Italy and Greece, absorb the vast bulk of the migration flows and responsibility.

The European project is very much an American project. Its failure will do great damage not just to Europe but to America as well.

There is no effective plan, let alone a Europe-wide agreement, to deal with a continuing and burgeoning refugee stream that is sure to outstrip the 1 million-plus people who entered Europe last year. Instead, every European country is trying to handle the problem on their own by re-instituting border controls that jeopardize the free movement of goods and people — the hallmark of the EU. The drive towards re-nationalization is reaching its zenith in the U.K., which is seriously contemplating leaving the Union altogether.

An inward-focused, self-absorbed, re-nationalized Europe constitutes a major threat to European security and the transatlantic relationship.

For 70 years, starting with the Marshall Plan, the U.S has been committed to help build an open, cooperative, and ultimately united Europe — whole, free, and at peace. Russia posed a major threat to that Europe when in 2014 it invaded Ukraine. ISIL similarly poses a critical threat to people’s safety and security. But a Europe that is no longer open, that shuns cooperative solutions for reliance on national ones, is a more lasting and significant challenge to the countries and people of Europe.

Many Europeans understand this challenge; and many are determined to counter it. But they need help, which a much more engaged America can and must provide. Washington needs to underscore the danger of the current re-nationalization trend, the need to forge a European-wide strategy for refugees and the necessity of Britain to remain in Europe. After all, the European project is also very much an American project, and its failure will do great damage not just to Europe but to America as well.

Ivo Daalder, former U.S. ambassador to NATO, is president of the Chicago Council on Global Affairs.