The longer Germany’s grand coalition stays in power, the more doubtful it is that Berlin will raise defence expenditure to the levels that it has promised its US and European allies. A failure to meet Germany’s commitments will corrode the mutual trust that is the essential ingredient of a successful international military alliance.

Inadequate German defence spending weakens Nato, the foundation stone of the nation’s security for 70 years. It damages US-German relations, which are at a post-1945 low because of the Trump administration’s disruption of the liberal world order. Ultimately, it undermines the credibility of the government’s claim that Germany stands for a robust, autonomous European security and defence strategy less reliant on Washington.

The Christian Democrat-Social Democrat coalition that assumed office in 2017 informed Nato at the start of this year that it would spend 1.5 per cent of gross domestic product on defence in 2024. This figure was below the 2 per cent to which Angela Merkel, Germany’s chancellor, and other national leaders had committed themselves at a Nato summit in 2014.

However, in a medium-term budget plan adopted in March, the government foresaw even lower defence expenditure, at a mere 1.25 per cent of GDP in 2023. Even if actual spending turns out a bit higher, Germany will remain the weakest military power among Europe’s largest countries despite having the strongest economy.

True, Germany is not the only offender. At present only seven of the 29 Nato allies — the three Baltic states, Greece, Poland, the UK and US — meet the 2 per cent target. Moreover, the UK, which on paper is Nato’s second-biggest defence spender, meets this goal thanks partly to creative accounting. Overall, European governments have expanded their defence budgets since Russia’s annexation of Crimea in 2014. But more will be necessary if the Europeans are to persuade the US of Nato’s continuing usefulness.

This argument applies with particular force to Germany. President Donald Trump’s displeasure with Berlin’s export surpluses and energy ties with Russia is overdone. On defence, however, it is not just the Trump administration but all its recent predecessors that have rebuked Germany.

Serious policymakers in Berlin know that the US is right. Germany’s federal government budget has been in surplus for the past four years. Nato membership gives Germany not only security but a good financial deal. Wolfgang Ischinger, a distinguished former diplomat, estimates that, if Nato did not exist, Germany would have to double its defence budget to 3-3.5 per cent of GDP or risk being “completely blind, deaf and defenceless”.

Some German politicians contend that Berlin’s involvement in Nato operations around the world shows that its contribution is greater than the low headline number for defence expenditure suggests. But the problem goes deeper. The Social Democrats, desperate to reverse their electoral decline, would rather promise more social spending than seek to persuade voters of the need for a bigger defence budget. In the Trump era the party is more than ever tempted to woo voters inclined to pacifism with the siren song of anti-Americanism.

It is encouraging that Annegret Kramp-Karrenbauer, Ms Merkel’s successor as CDU leader, has spoken in favour of raising defence expenditure above the government’s plans. But if she wins the next election, due by 2021, and is serious about achieving this goal, she will have to consider ending the coalition with the Social Democrats.