What began as a spat between Friedrich Merz and Donald Trump over the Iran war is rapidly turning into a historic rupture between Germany and the US. Its significance is hard to overstate. In Germany, the transatlantic falling-out adds to the domestic woes of a coalition government in crisis, overshadowing the first anniversary of Merz’s becoming chancellor tomorrow.

More importantly, it proves the futility of Merz’s attempt to be Europe’s Trump-whisperer and puts Nato’s credibility into question. But the dispute also boosts the ambition that Germany’s conservative leader set out on the night of his party’s election victory: to make Europe more independent from the US security umbrella.

This rupture started with the chancellor’s remarks last week about the stalling talks between the US and Iran. The Iranians, Merz observed to an audience of secondary schoolchildren, “are obviously very skilled at negotiating, or rather, very skilful at not negotiating, letting the Americans travel to Islamabad and then leave again without any result”.

“An entire nation,” he concluded, “is being humiliated by the Iranian leadership.”

It was an unexpected moment of truth-telling, one that may turn out to be rather costly. Merz has replaced Pope Leo as the favourite target of Trump’s late-night social-media invective. The chancellor, Trump posted, “doesn’t know what he is talking about”, is “totally ineffective”, is presiding over a “broken country” and presumably “thinks it’s OK for Iran to have a nuclear weapon”.

To state the obvious, Merz does not think that would be OK. In the Oval Office with Trump in March, Merz had explicitly agreed with Trump’s war aim of getting rid of the Iranian regime. He declared it was “not the time to lecture our partners and allies” on international law. Germany did not close its airspace, nor even consider restricting the use of US bases on its territory.

Stay on the sidelines, offer post-conflict assistance, but do not poke the bear – that had been the German policy. So why did Merz suddenly decide to go straight for the jugular of Trump’s narcissistic self-image – his supposed dealmaking prowess? Was it just one of the impulsive comments the chancellor is notorious for?

There may be more to it. Merz is not backing down. In a long interview on primetime TV on Sunday, the chancellor tried a more conciliatory tone, but he did not walk back his remarks, despite being pressed several times.

This is all the more remarkable because severe punishments are incoming: 5,000 US troops are to be withdrawn from German bases – maybe even “many more”, a furious Trump hinted at the weekend. No Tomahawks and other mid-range missile systems will be stationed in Germany, despite a standing agreement dating back to 2024. That deal was supposed to close a dangerous deterrence gap with Russia, which has stationed nuclear-capable missiles in Kaliningrad, threatening European capitals. European countries are working on their own capabilities to counter Russia, but they will come online in six to eight years at the earliest.

Taken together with the troop withdrawals, the cancellation of the missile battalion significantly weakens Nato’s position. Deterrence, after all, is a mind game; it depends on credibility and political will. Nobody in Germany’s strategic community thinks it is a coincidence that Trump’s announcements came after he had another “productive” 90-minute phone call with Vladimir Putin.

On top of all that, European carmakers are being saddled with a 25% import tariff, effective immediately, according to Trump – which obviously will harm German industry most.

Merz’s comments, while certainly a departure from diplomatic protocol, hardly warranted such retribution. After all, it was Merz who set in motion a big rearmament drive for the Bundeswehr, dispensing with Germany’s constitutional debt brake to finance it. This was exactly the burden-shifting in conventional defence the US had long called for.

In Europe, Germany has taken over most of the financing of Ukraine’s military support. It was Merz who pushed for the 5% of GDP spending pledge at the Nato summit last year, so Trump could claim he got the Europeans to pay more for collective defence. And it was the German chancellor who put pressure on the European Commission president, Ursula von der Leyen, to accept Trump’s unfair trade deal, notwithstanding the disadvantage it imposed on German exporters. Even when Trump threatened to invade Greenland, Merz argued for a calm European reaction.

The folly of the war against Iran has turned out to be the last straw, however. Soft-pedalling is no longer possible. This war is undermining European security. Air defence systems urgently needed by Ukraine (and on Nato’s eastern flank) are diverted to the Middle East. While Kyiv has to beg for every single Patriot interceptor, more than 1,000 have been used against Iranian attacks.

The consequences of the US-Israel “excursion” also threaten to sink the German economy, and with it Merz’s urgent domestic reform agenda. The already meagre growth forecast for 2026 has been slashed by half because of the war. This adds stress to the already fraying coalition of conservatives and Social Democrats.

But it might also help them find new purpose. For a year now, Merz has tried to manage Trump through concessions and appeasement. The failure of this policy is the real reason Merz snapped in front of the students.

One thing has become even clearer from Trump’s angry retribution: dependence on a US administration that punishes its allies while accommodating Europe’s enemies is untenable.

More than a year ago, on the night of his party’s victory in Germany’s general election, Merz said he wanted to create unity in Europe as quickly as possible, “so that, step by step, we can achieve independence from the US”. Those words ring even truer today.

• Jörg Lau is an international correspondent for the German weekly Die Zeit