Donald Trump recently ignited yet another firestorm by hedging when asked whether protecting the newest NATO member, tiny Montenegro, might be worth risking a war.

Of course, the keystone of NATO was always the idea that all members, strong and weak, are in theory equal. A military attack against one member, under Article V of the NATO charter, meant an attack on all members.

Such mutual defense is the essence of collective deterrence. An aggressor backs off when he realizes his intended target has lots of powerful friends willing to defend it.

But what happens when an alliance becomes so large and so diverse that not all of its members still share similar traditions, values, agendas or national security threats?

NATO’s original European members considered themselves kindred neighbors under the nuclear umbrella of the United States.

With the inclusion of West Germany in 1955, NATO’s original mission was altered somewhat. It was no longer tasked just with keeping the U.S. in and the Soviet Union out, but also with raising Germany up rather than keeping it down.

NATO collective defense was designed to offer breathing space against the superior forces of the Soviet Red Army — until the United States could bring in reinforcements or threaten to use its superior nuclear forces against would-be aggressors.

The alliance worked because the United States accepted that Europe needed American help to deter enemies in order to avoid repeats of the disasters of 1914 and 1939. With the exception of Turkey, the older members of NATO were generally seen as sharing the geographical space of Western Europe.

That is no longer quite true. Many of NATO’s newer members are not integrated into Western Europe. They are now spread all over the continent, and they include former Russian allies such as Albania, Bulgaria and Montenegro. Many of the newer members are small, vulnerable and in crises would need far more help than they could provide others.

The idea of NATO has changed as well. Instead of deterring a Soviet invasion of Europe while rehabilitating Germany, NATO has become less a defensive military alliance and more a de facto cultural institution to homogenize Europe along Western lines.

For some in Europe, NATO is envisioned not so much as a collection of planes and tanks, but instead as an expanded version of the European Union.

The more diverse NATO has become, the less unified it has become, especially with the demise of the original threat of the Soviet Union. As post-Cold War Europe grew calmer and more affluent, NATO members became less likely to believe that they would ever need to sacrifice to invest in their mutual defense.

In the aftermath of the Cold War, NATO was eager to enlist eager Eastern European and Balkan nations that rightly had feared Russia even after the end of the Soviet Union.

But southeastern Europe and the Balkans were also home to age-old feuds and surrogate wars between rival empires — from World War I to the Bosnian War in the early ‘90s.

What are the lessons of NATO expansion?

One, vastly increasing its membership can only make NATO weaker, not stronger. In some sense, when everyone is in an alliance, no one really is. Vladimir Putin may gamble to find out whether affluent Dutch or Belgian youth will really be willing to die fighting for the territorial integrity of distant Bulgaria. If not, then Article V will be exposed as a farce and NATO itself will be finished.

If Albania and Montenegro are in NATO, why not Austria, Finland, Kazakhstan, Macedonia and Serbia? Will Mexico join Canada and the U.S. to round out the North American membership?

Two, the borders of the “North Atlantic Treaty Organization” are now ill-defined.

Recep Tayyip Erdogan’s Turkey is becoming an updated version of the old Islamic Ottoman caliphate. It is an enemy of the Kurds and Israel, both staunch U.S. allies. If Turkey gets into a “defensive” conflict with Israel, would young soldiers from Kansas want to risk death to “defend” an anti-American, authoritarian NATO theocracy from a pro-American liberal democracy?

Tough decisions, not more weary and sanctimonious rhetoric, are needed to revitalize NATO.

The alliance must insist that all members quickly meet their military obligations of spending 2 percent of their GDP on defense. If a rich country in peace reneges on its promise of military readiness, why would anyone expect it to fulfill its pledge of assistance in wartime?

NATO should insist on common values and agendas, and its members should formally identify their likely collective enemies.

The alliance must ensure that any nation in NATO belongs in NATO — and thus is worth risking what could become a nuclear war on its behalf.

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