NATO’s key attraction is the same as Mount Everest’s. When Edmund Hillary, the first to reach the top, was asked, why he wanted to scale the world’s highest mountain, he famously answered: “Because it is there.”

The North Atlantic Treaty Organization is celebrating its 70th birthday this month. It has been “there” longer than any alliance of free nations in history. Admittedly, this is technically not quite correct, given the Anglo-Portuguese Alliance of 1386 still on the books today. Effectively, Britain-and-Portugal has been superseded by the countries’ integration into NATO.

“Being there” at such an advanced age as 70 is an extraordinary feature of world politics. Normally, alliances expire when they triumph. The Soviet-American one against Hitler’s Germany was dead less than one year after V-E Day 1945. The World War I coalition against the Kaiser expired soon after the Versailles settlement, when Britain went back to balancing against its ancient rival France while Germany and Soviet Russia joined forces against the West at Rapallo in 1922. And so it went through the ages.

The explanation is simple. When nations-in-alliance beat their foe, they start worrying about each other, just as the United States and Russia, the two real winners of WWII, did at the outbreak of the Cold War. So a reversal of alliances is a natural adjunct of victory as nations recalculate the balance. Or think of the Sunni powers tacitly aligning with Israel against Iran, never mind the many wars Arabs and Israelis had fought in the past. Now revolutionary Iran is the greatest threat to the region.

To note these dynamics makes NATO’s longevity even more astounding. Just as surprising is its expansion. In the beginning, twelve nations signed up. Now, the number has grown to 28, including the former Warsaw Pact countries (minus Russia). So longevity and growth suggest not just endurance, but also functionality. Even in those halcyon years between the collapse of the Soviet Union in 1991 and the resurgence of Russian ambitions under Vladimir Putin circa 2008, the Alliance held. Instead of shrinkage in the absence of an overweening threat, NATO gained new members.

How to explain such a startling turn that defies historical experience?

The mightiest factor, of course, is the United States. Realism bids us to concede that NATO at heart is a unilateral security guarantee, never mind that the Europeans field 1.5 million soldiers (at least on paper). Though Europe has as many armed forces as does America, the United States protects Europe, not vice versa. Nuclear deterrence is Made in the U.S.A., what with thousands of strategic warheads that dwarf the modest potential of Britain and France.

With America in play, the Europeans enjoy a broad margin of security at a significant discount. Safety on the cheap breeds loyalty instead of betrayal, especially among the smaller countries whose armed forces would not even fill a soccer stadium. The United States built the house, and its allies have been paying subsidized rent. Germany spends as as much on defense as a share of GDP (1.25 percent) as tiny Croatia.

An oft-neglected factor is deeply political. Why did ancient rivals like Britain, France, and Germany come together after centuries of rivalry and bloodshed? Why are arch-enemies like Greece and Turkey in the same alliance? Why did the smaller members get in bed with the strong who in the past invaded and swallowed them? Because of Uncle Sam in the guise of Mr. Big.

Suddenly, there was a player in the game bigger than each and all. He could protect his wards not only against Russia, but also against each other. That delivered trust, the scarcest commodity in the affairs of nations. France and Germany would never have linked hands across the Rhine. But once U.S. divisions were stationed in Europe, the ancient calculus of fear and suspicion changed in a benign direction. Indeed, France fought tooth and nail against West German rearmament in the early 1950s until the United States committed six divisions to Western Europe. For once, these two, who had fought three wars in a single lifetime (1870, 1914, 1940), could cooperate because America insured both of them against the perils of misplaced credulity.

If the arrangement was so profitable for the Europeans, what was in it for the United States? The tally is quite impressive. For one, the U.S. commitment kept the world’s most critical strategic asset out of the hands of the Soviets. Today, the same interest obtains in the face of Russia’s neo-expansionism under Vladimir Putin. “Just being there” deters Russian adventurism. Keeping 500 million people and the world’s second-largest GDP out of Mr. Putin’s claws is obviously a permanent American interest. What’s good for Europe is good for the United States. Hence watch Donald Trump’s actions rather than his bluster.

His bashing of NATO (“obsolete”) since 2016 would fill a medium-sized book by now. At the core of his litany is his constant attack on Erope’s free-riding, picking up where Barack Obama left. “Free riders annoy me,” Obama told The Atlantic. The difference? Obama cut U.S. forces in Europe, while Trump is bolstering the American commitment.

U.S. Special Forces are training Baltic soldiers in the arts of guerilla warfare.

In a clear signal to the Kremlin in 2018, the United States demonstrated its ability to dispatch heavy weapons to Europe, including 87 Abrams tanks and some 500 armored vehicles. 1,500 troops are being deployed to Germany. America’s 4,000-strong military presence in Poland is slated to grow. After 2005, the fabled Sixth Fleet dwindled to one ship permanently assigned to the Mediterranean. Today, Washington is planning to deploy a carrier force to the Med as a message to Moscow. The force would allow other Navy to ships to patrol the Baltic and the Black Sea.

If President Trump were really out to deconstruct NATO, he would not commit men and material to the European Reassurance Initiative. The gist of the matter is this: West European capitals fear and despise Trump, and he pays them back in spades with his threats and insults. Yet the hard facts show that the Old Lady has a lot of vigor in her at age 70. The Europeans started boosting defense spending before Mr. Trump entered the White House. Even the peace-minded Germans are increasing defense expenditures—no wonder when half their tanks (down to 280 from 2,500) are out of commission, and so are all of their six U-boats.

Yet in contrast to Mr. Trump’s badmouthing, NATO has not been idle in the face of new threats. The NATO Response Force (NRF) has grown from 13,000 to 40,000 troops. A Very High Readiness Joint Task Force (VJTF)—a multinational brigade—should be ready to deploy within two to seven days. NATO now exercises more frequently than in the past 20 years. Trident Juncture 2018 in Norway and Iceland, in the North Atlantic and the Baltic Sea, saw 50,000 troops in action, as well as 250 aircraft, 65 naval vessels and 10,000 vehicles.

NATO members have deployed one multinational battle group in Estonia, Latvia, Lithuania and Poland as part of the Enhanced Forward Presence (EFP). These groups are led by Canada, Germany, the UK and the United States. These “tripwires” were once strung out along the intra-German border; today, they serve the same function on the eastern edge of the Alliance, which has also committed to beefing up heavy reinforcement forces. All told, European NATO members  have increased total defense expenditure by 13% in real terms from 2014 to 2018.¹

Those who wring their hands over the impending demise of the Alliance, might also take solace from the past. Basically, the history of NATO has been the history of its crises, which can fill another book. Just to pick one egregious example: Under De Gaulle, the French pulled out of NATO’s integrated force structure. By 2009, they were back.

The explanation, to repeat, is “functionality.” Nations don’t ditch what has worked well for them in the past, no matter how turbulent their relationship. Alas, there are two caveats. First, Europe might organize its own defense and so render NATO indeed obsolete. Second, the United States might pull back into Fortress America. The first outcome requires turning the 27 members of the EU into an e pluribus unum. The second presumes America’s willingness to vacate its exalted perch at the top of the global hierarchy. Both might happen.

Hard-boiled realists, however, should bet on the Alliance celebrating its hundredth birthday in 2049.