After the American drone assault on Qasem Soleimani, Tehran’s de facto No. 2, EU Council chief Charles Michel predictably tweeted that the “vicious circle of force, provocation, and retaliation. . . .must stop.” Resist escalation, whose flames would engulf the entire Middle East, ran the counsel. In short: Count us out.

Germany’s Foreign Minister, Heiko Maas, pledged to wield “all levers” to squelch escalation. Berlin would talk to everybody, “including Iran.” He pressed the United States to respect the Iraqi parliament that demanded the withdrawal of U.S. forces from the country, calling Donald Trump’s threats against Iran “not very helpful.” To get out of harm’s way, Germany will “thin out” and redeploy its mighty force in Iraq, all 120 of them—instructors, not combatants.

Though Britain, according to its Foreign Secretary, was “on the same page” as the Americans, Prime Minister Boris Johnson would work with the EU to urge de-escalation. French President Emmanuel Macron called for “restraint” on all sides. Trump’s European friends, the Poles, also sought to “defuse tensions.” NATO head Jens Stoltenberg, normally ready to flex the Alliance’s muscles, reiterated the European party line: “The important thing now is to de-escalate.”

Calming the waters, of course, is what diplomats do; it’s in their DNA. But as evenhanded go-betweens, the Europeans are out of their depth, which is “no accident, comrades,” as the Soviets used to say. Unless they want to be ignored or outflanked by the principals, arbitrators must be equipped to weigh in with power. To succeed, peacemakers should be able to threaten spiteful antagonists with dire consequences or at least with the denial of support. Lacking such clout, the Europeans plead, warn, and cajole.

Mr. Trump is not impressed, and neither is Ali Khamenei, the Religious Leader. He did not go for a merely a symbolic strike against U.S. forces in Iraq in order to please the Europeans. Whatever happens next will also unfold in the context of American escalation dominance.  The United States has allies throughout the region; Iran has only tiny Qatar. The United States has bases all around; Iran does not even have one in Cuba.

Caught in the middle, the EU is no dwarf who would be reduced to empty-handed posturing between the two antagonists—at least not on paper. Start with friendless Iran. Its economy is tottering, so the EU could threaten severe sanctions to save whatever is left of the nuclear deal (JCPOA). Tehran now wants to shred it, by going for uranium enrichment toward weapons-grade levels. Europe wags its finger only softly. Angst edges out resolve because punitive sanctions, as the catechism has it, would actually accelerate Iran’s march to the Bomb.

The EU might also field a credible fleet to deter Iran from sinking tankers in the Gulf. But with one French ship and two UK vessels? Britain had some 250 major surface combat vessels in World War II; now it is down to 20. Germany has 14—not a blue water navy, but one configured for the Baltic and the North Sea. Berlin has already bowed out of the Gulf flotilla; thank you for the invitation, Mr. Trump. Their well-considered interests would urge the Europeans to secure the energy lifeline passing through the Gulf. Yet the lack of naval strength undercuts strategic logic. Add sheer fear, given that Iran can hit back asymmetrically with a terror campaign in Europe.

Now turn to Donald Trump. How would Europe keep him in line? The EU will not inflict sanctions on the globe’s mightiest economy. Would the Europeans deny the United States basing and overflight rights and so separate itself from the superpower that still guarantees their security? There is no love lost between Macron, Merkel et al., and Trump; indeed, alienation courses ever more strongly through the Alliance, the world’s oldest. Still, fear and loathing will not cut through the dependence that chains Europe to its patron from across the sea.

Let’s set aside the frayed friendship and invoke hard-nosed realpolitik. It might counsel the Europeans to embrace Trump in order to leash him, to hold him close to keep him from running off the cliff. Add the commonality of interests toward Iran, such as stopping its expansion across the Middle East, its nuclear ambitions, and the deployment of surrogates against Western allies like Israel and Saudi Arabia. Alas, siding with Trump is a fool’s errand even for America’s best friends. 

The President does not reward loyalty. Instead, he breaks commitments with the abandon of a small child who has not yet learned the virtue of promise-keeping, let alone the value of reciprocity. Nobody in his right mind would get in bed with a man who will reverse himself in the space of two tweets. Take Iran as a case in point. 

Trump did not move after Iran’s attack on Saudi oil fields last year, leaving his best Arab ally in the lurch. He did not retaliate against the downing of a U.S. drone. And suddenly, out of the blue, he kills Soleimani. True, the general was the mastermind behind Tehran’s expansion all the way to the Mediterranean. Yet he has been in American sights for years while racking up deadly hits on U.S. troops by his stand-ins. Great powers should not abruptly switch from pussycat to tiger instead of serving a long-term strategic design. That’s exactly what Trump did, though, stunning friends and foes alike. 

How about a summit with Ali Khamenei? Trump has done it before; recall the Kim Jong-un caper. Or beefing up U.S. troops as prelude to a pull-out to end America’s “endless wars” in the world’s oldest arena of great power rivalry? Lest we forget, this is a familiar American habit, as both George W. Bush and Barack Obama zigzagged between “surges” and draw-downs, never mind those who had joined their fates to the United States. The pattern transcends the antics of Donald Trump. A remake on his part would leave America’s allies, who have committed forces to the region, with nothing to chew on but the bitter fruits of their gullibility. 

America, thy name is capriciousness; but this is only one reason why even its best friends hang back from Trump’s mano-a-mano with the Khameinists. The fundamental fault line is drawn by Europe’s impotence. To crib from Thucydides, the strong do because they can, and the weak don’t do what they should. Theoretically, the Europeans should join the United States in the containment of Iran. The West and the Sunni states are facing a revolutionary, not just revisionist, power. Revisionists merely want to increase their pile of chips; they can be swayed by give-and-take in the European way. On a mission from God, Iran’s revolutionaries want to overturn the table and demolish the casino. In the end, they must be defanged by superior force, as was Napoleon, or by awe-inspiring deterrent strength, as was the Soviet Union. 

In their hearts, EU leaders know the difference between a revisionist and a revolutionary power. Yet in spite of their fabulous assets, they will not rekindle their great power vocation. Why would they, when two world wars have ended their imperial careers while America gifted them security at a steep discount? 

So, in the latest U.S.-Iran clash, the EU has acted rationally. Wisely, this rich and risk-averse “civilian power” has distanced itself from the hit-and-run man in the White House who will betray partners in a day’s tweets. Burned so often by American hauteur, a strategic lightweight like the EU cannot but resort to suasion, mediation, and de-escalation to evade entrapment in a conflict it cannot control. “Honi soit qui mal y pense,” runs a famous French phrase: Shame on those who think evil of it. Don’t blame the weak for choosing discretion over valor when they cannot stand up to the bullies of this world.