On the night of November 13th, the eyes of the world were drawn to Paris by the horrors unfolding there. How vulnerable France seemed in the face of these terrorists bent on mass murder. But the need to deal with this mortal threat could form the basis of a new Anglo-French alliance. In the fight against ISIS, David Cameron is keen to make common cause with President Francois Hollande.

For Hollande, the attacks have offered, paradoxically, a chance to change the subject. In place of the unresolved disagreement with Berlin about how to save the euro came the imperative requirement to speak for the French nation in its hour of danger. Hollande declared France to be at war. He set out to mobilise an international coalition which will “eradicate” ISIS, and he told the European Union to stop lecturing France about the budget deficit. Economics now takes second place to security. Here is a subject on which Paris can be more dynamic than Berlin. Hollande is spending the first four days of this week trying to mobilise Britain, the United States, Russia and Germany against ISIS.

If Hollande had failed to react in this way, he would have found himself outflanked by his opponents on the Right, including Marine Le Pen, the leader of the Front National, profiled at the start of this year on ConservativeHome. As it is, the President’s approval ratings have risen, and this at a time when sales of the tricolour have soared to levels last seen in 1998, when the French football team won the World Cup, and before that in 1970, at the death of General de Gaulle.

Defence is a natural field for close co-operation between Britain and France. Many in London have rediscovered, in the present crisis, a romantic love of France as a grand nation: a welcome companion in arms against ISIS in Syria. But as the Libyan venture reminded us, Anglo-French co-operation does not always lead, on the ground, to the desired result: a lesson illustrated with humiliating clarity at Suez in 1956.

And while many Britons will be delighted to make common cause with the bellicose Hollande, and will contrast his response favourably with the complete failure of Jeremy Corbyn to strike the right note, we should not allow this to obscure the plain fact that France’s most important relationship since the Second World War is with Germany.

And that relationship is in deep trouble. As Charles Grant, director of the Centre for European Reform, put it to me earlier this week: “The lack of trust between them is enormous.” At issue is how to make the euro work. In Grant’s words, “The future of the EU means fixing the euro, and the French have a plan, and the Germans reject it. Nothing else, e.g. Brexit, matters a lot. The French understand that the euro may be doomed unless the Germans shift their position, which they are not doing.”

Grant has recently published a learned account of the design flaws, stretching back 25 years, which mean the euro has not worked as well as its architects hoped.

The restraint with which he writes, his immersion for many years in the subject, and his generally pro-European attitudes, make his last two paragraphs all the more striking:

“My biggest worry for the future of the euro is the intellectual isolation of much of Germany’s financial elite from the rest of the world. The problem is not so much that German policy-makers are wrong on everything – for example, they are right that structural reform is essential and that Keynesians can over-prioritise the short term – but rather that some of them think they have little to learn from others. I have heard senior German figures speak of Southern European, French or Anglo-Saxon economic analysis contemptuously. I have also heard them refuse to consider the eurozone’s overall fiscal stance, while insisting that the German, French and Italian economies be treated as separate entities.

“What the eurozone needs are not federal institutions – desirable though they might be – so much as a Germany that is more sensitive to its partners’ needs, less arrogant in dealing with them, more open to others’ economic thinking, and more willing to acknowledge that the eurozone economies all affect each other.”

This situation is a nightmare for the French. They wanted the euro because they were fed up with being dominated by the German mark. But now that they have the euro, they find themselves as powerless as ever to control their mighty neighbour. German policy-makers hand out moral lectures about the need for austerity: an austerity which has deepened the debt crisis among the southern members.

In vain the French have called for a more Keynesian approach, with mutualisation of debt, and the EU budget used to subsidise the poorer members. For members of the Bundestag, this would mean admitting to their own taxpayers that they the Germans are, after all, going to have to subsidise the euro’s weaker members: in political terms, a disastrous message.

The natural response of the German political class is to insist, in a blinkered and self-righteous manner, that if only everyone obeys the rules, all will be well. But since everyone, including the Germans, breaks the rules whenever it suits them, these strictures lack credibility.

In January 2013, David Cameron said in his Bloomberg speech: “Those of us outside the euro recognise that those in it are likely to need to make some big institutional changes.” Yet nearly three years later, it is still not clear what changes the euro’s members will be able to agree.

Nor is it clear how France can flourish in the euro. Writers such as Eric Zemmour suggest that since the death of de Gaulle, the French nation has been committing suicide. Edmund Burke thought the problem had its roots two centuries further back, in the Revolution, when the French tried to extinguish a part of themselves.

What is Hollande to do? He has had the sense to abandon the extravagantly socialist economic policies which aroused Ed Miliband’s admiration, and encouraged many of the well-to-do French to seek a better life in London.

But on European policy, Hollande sees the British doing all they can to cultivate the closest possible relationship with Germany, as the one country that can deliver sufficient concessions to avert Brexit – or concessions that Cameron hopes will be sufficient. From Berlin, one never hears the view that the EU would be much better off without the British. From Paris, angry editorialising along those lines is sometimes heard. One wonders how long the solidarity inspired by the Paris bombings can sustain Anglo-French unity.