As Donald Trump’s second term has become overshadowed by plunging poll ratings and an illegal, ill-advised war in the Middle East, European governments have regularly been singled out to bear the brunt of the US president’s growing frustration. Sir Keir Starmer’s refusal to militarily back the attack on Iran led to unfavourable comparisons to both Winston Churchill and King Charles. “Unfriendly” Spain has been threatened with a trade embargo for similar reasons. Italy’s prime minister, Giorgia Meloni, previously seen as a key political ally, has also been on the receiving end. “I’m shocked by her,” Mr Trump said last month. “I thought she had courage. I was wrong.”

Currently it is Germany’s chancellor, Friedrich Merz, who finds himself in Washington’s crosshairs. In the wake of Mr Merz’s accurate observation that the US has no convincing strategy on Iran, the Pentagon has announced the future withdrawal of 5,000 US troops from bases in Germany. Vital long-range weapons are also to be withheld as American military stockpiles are depleted by events in the Middle East. For good measure, Mr Trump has threatened to raise tariffs on European car manufacturers to 25% – a measure that would hit Germany hardest.

At the weekend, Mr Merz was at pains to stress that he was not “giving up” on either the transatlantic relationship or his own with Mr Trump. Given the stakes, he was right to do so, though the US president’s thin skin, vengeful instincts and overt hostility to the EU mean that the next flare-up will never be far away. But the latest evidence of Washington’s desire to disengage from its postwar role as the guarantor of Europe’s security should have ramifications beyond diplomacy.

In a changed world where old assumptions no longer apply, the potential shape of future European defence cooperation has gradually become clearer over the past year. The French president, Emmanuel Macron, has said that France’s strategic nuclear deterrent could be extended across the continent. The EU has meanwhile made €150bn available in cheap loans for security spending.

But when it comes to strategic priorities and procurement policies, too much of the picture remains fragmented along national lines. Mr Merz has continued the direction of travel initiated by his predecessor, Olaf Scholz, who announced a huge increase in defence spending following Russia’s invasion of Ukraine. But tensions persist over the extent to which Berlin should buy European, and the implications of Germany – the EU’s biggest economic power – also becoming its military hegemon.

The way to defuse them is by making more resources available at an EU level through joint borrowing, ensuring that this money is used to boost European manufacturers, and deepening cross-border collaboration over how it is spent. From the green transition to migration and security, the 21st-century challenges that Europe faces must be collectively confronted, if they are to be successfully addressed.

Mr Merz has hitherto displayed the traditional German aversion to sanctioning more EU debt, while relaxing domestic rules on borrowing in order to finance greater defence spending. That may make short-term political sense, as he seeks to manage his increasingly fractious coalition government. But the latest round of warnings and threats from Washington has underlined the already obvious: a new model of European security for a new era must have solidarity and joint decision-making at its core.

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