Migrants wait to disembark from the Italian Coast Guard ship Fiorillo, at the Catania harbor, Sicily, southern Italy, Friday, April 24, 2015. (Alessandra Tarantino/AP)

Jim Hoagland is a contributing editor to The Post. His e-mail is jim.hoagland@washpost.com.

The Cold War made Europe’s Atlantic identity paramount in the Old Continent’s dealings with the world and particularly with its American ally. Mediterranean and Central European countries developed habits of cooperation and consultation in NATO that promoted the flow of security commitments, commerce and ideas across the North Atlantic.

But the turmoil sweeping the southern and eastern shores of the Mediterranean today threatens to transform Europe’s strategic outlook on security matters. The United States must pay careful attention to the growing gaps in the ways Washington and its European partners define the greatest dangers to their common well-being.

“For us, the biggest threat comes from the South,” says an Italian friend deeply experienced in NATO matters who now spends his time worrying about waves of migrants fleeing the wars, poverty and breakdowns in governance along the Mediterranean littoral. “Our nightmares are not about Russian tanks invading from the east. They are about the terrorists a short boat ride away in Libya.”

“Nobody in France is debating about arming Ukraine,” a conservative French parliamentarian once known for his hawkish Cold War views told me this week. “We are debating how much national surveillance we need to spot terrorists returning from war zones in Syria and Iraq, and how to stop Africa from completely imploding.”

Many Americans will be tempted to leave the Mediterranean crisis to the Europeans to deal with on their own. But the need for a new emphasis on alliance solidarity and burden-sharing is underscored at this moment by Vladimir Putin’s aggression in eastern Ukraine and the uncertain NATO response it has provoked.

NATO is now, in fact, divided into three factions on Ukraine.

Mediterranean countries are distracted and distant from any common effort to oppose Putin’s attempt to rewrite the rules of war and peace in modern Europe in Russia’s favor. Fatigue with economic sanctions against Russia is setting in for France, Italy and even Britain. (One measure of the decline of the Atlantic identity in European affairs is Britain’s growing absence in power projection and major-power diplomacy. The current election campaign has contributed to this tendency, but larger forces are shaping the pullback.)

Poland and the Baltic nations, fearing they could be next on Putin’s list, are at the other end of the spectrum. They take a tough, Atlanticist line and seek to deter Putin by multiplying the tripwires that would bring NATO troops to their aid in the event of Russian attacks across their frontiers.

The Obama administration occupies a shifting, reactive middle ground on Ukraine, letting German Chancellor Angela Merkel play the leading role in dealing with Putin while trying to reassure Poland and the Baltic states by rotating U.S. planes and troops into their territories in an operation code-named Atlantic Resolve.

This approach has the merit of keeping Russian-American tensions at a manageable level. But it has not halted Putin’s buildup of Russian forces in Ukraine or prompted him to rein in military commanders who threaten the West with nuclear destruction when the whim strikes them. It will be surprising if the Russian leader does not see a golden opportunity here to drive a wedge between the United States and Europe and if he does not keep the opportunity alive as long as he can.

That alone is a strong reason for Americans to show solidarity with European allies confronting a traumatic redefining of the security challenges to their societies. This is particularly true of Italy, which has been left alone to cope with the swelling tide of refugees and migrants over the past four years.

Italy is likely to see 200,000 migrants come ashore this year, Prime Minister Matteo Renzi told me this month in Washington, where he appealed to President Obama for U.S. support for European efforts to handle that tide. European Union leaders pledged ships and other help at a summit Thursday.

Washington should now see to it that NATO’s Atlantic Resolve mission to bolster Ukraine’s neighbors is matched by alliance involvement in the Mediterranean crisis. That involvement — call it Mediterranean Resolve — would take a different form, but it would also demonstrate that security is indivisible for all NATO members. The forces driving desperate people from Eritrea, Syria, Mali and other failing countries to put their fate in the hands of human smugglers in Libya are global ones that require a coordinated response from history’s most successful alliance.