LONDON • It's easy to see why the British lavished their best hospitality and all the military pomp they could muster on French President Emmanuel Macron, who visited their islands last week; British Prime Minister Theresa May needs all the help she can get in getting a good deal out of her country's separation from the European Union, and few people are better-placed to facilitate this than Mr Macron, Europe's strongest and most respected European leader.

But it's not evident why Mr Macron should pay much attention to his British neighbours, since the French President's top priority is to revive and boost France's much closer and more substantial alliance with Germany, which really acts as Europe's ultimate pillar and decision-maker.

And yet Mr Macron did invest a great deal in his British trip; he came with a retinue of no less than seven top ministers, accepted everything Mrs May asked of him, including giving the British premier an electoral boost by visiting her parliamentary constituency, and signed a raft of a new cooperation agreements.

There is actually a very simple explanation for this behaviour: whatever happens to Britain after it leaves the EU, Britain and France will continue to need each other when it comes to defence matters, and will remain Europe's key military powers.

The real question is whether the Franco-British military link will remain a purely European instrument, or can acquire a bigger significance on the world stage, particularly in Asia. And on that, neither Mr Macron nor Mrs May has an immediate answer.

SHARED BLOOD, SEPARATED BY SEA

The French and the British frequently like to tease each other; an old British joke used to claim that the best thing between the two countries is the sea. But the reality is that the two nations have not been at war with each other for two centuries, an achievement France cannot boast of with any other of its neighbours.

During the 20th century, the two nations have repeatedly shed blood for the same cause. You won't have to drive for long through northern France before encountering a British military grave, always immaculately kept, and frequently adorned with fresh flowers laid by children from the local French school. And the Franco-British bond eventually extended throughout what was the British empire; just think of Singapore's own Clemenceau Avenue, named after France's World War I leader.

Of course, the two countries also frequently diverged. The British clung to the idea that their "special relationship" with the United States would always be far more important than any European enterprise, while the French turned to the Germans to build a European structure which is today's EU.

Still, many factors kept France and Britain together. Both are permanent members of the United Nations Security Council, a position which may seem today a historic anachronism, but not one either the French or the British are going to relinquish.

Both are nuclear powers. Both have sizeable conventional forces, the only European militaries capable of a long-term deployment outside their borders.

The economies of both countries are still among the top 10 in the world; their combined nominal gross domestic product is about half that of China's and about equal to that of Japan and South Korea put together.

Even more importantly, political elites in both countries continue to think in global terms. With very rare exceptions, politicians in most European capitals have no opinions about what should be done in the Middle East, what the rise of China may mean for them, or how other global problems should be treated. But in London and Paris, such matters are discussed not only in the corridors of power; they are also debated in the media and the voting public.

Furthermore, there is ultimately no substitute for their mutual defence collaboration. British military planners continue to put their faith in the North Atlantic Treaty Organisation or Nato, the US-led military alliance in Europe, but they are increasingly forced to admit that as long as President Donald Trump remains in the White House, US support for European defence projects will remain unpredictable, to put it mildly.

Domestic financial pressures also point towards further British cooperation with the French; secret protocols to the 50-year defence treaties concluded between Paris and London in 2010 envisage the mutual development and testing of nuclear weapons, for instance.

In 2010, both countries signed defence treaties to last 50 years - one on military and the other on nuclear cooperation. The treaties envisaged joint programmes on naval warfare, research on fighter aircraft, and the joint deployment of ground troops. The day-to-day partnership between the militaries is said to be "intense".

France has also recently discovered that, when it comes to defence, it is Britain rather than Germany which remains its key partner. The German public continues to be supremely unconcerned with defence issues, and the country's politicians see no reason why this should change. Promises which Chancellor Angela Merkel made to Mr Trump to increase Germany's defence spending have already been ditched, as part of Dr Merkel's recent coalition agreement with the centre-left Socialists.

And, as astonishing as this may seem, foreign policy matters did not even figure once in the coalition negotiations in Berlin. It is clear therefore that, for the foreseeable future, the defence of the European continent will largely be led by Britain and France; last week's public reaffirmation of this alliance by Mr Macron and Mrs May was, therefore, an admission of this basic fact.

ROLE BEYOND EUROPE?

But the implications of this strengthening alliance are yet to be worked out. As long as Britain remained in the EU, the French idea was that the military alliance with the British complemented France's political and economic alliance with Germany, putting Paris at the centre of most decision-making in Europe.

But with Britain leaving, that is no longer feasible; the British expect France to reciprocate by protecting British interests in Europe, or by at least ensuring that Britain's trade with the EU could continue largely unhindered and that, in turn, may anger the Germans. The political price which France is expected to pay to retain London's military alliance is, therefore, rising.

Nor is it very clear whether the Franco-British defence link will help Britain resist the temptation to split from Europe's foreign policy priorities once it is out of the EU. Mr Macron evidently hopes that will be the case, but there will be many countries around the world - China, but also the US - which will have an interest in tempting Britain to adopt different stances from those of the rest of Europe, and it is doubtful that Paris would be in a position to prevent this.

However, the biggest question is whether this Franco-British security link could play a broader international role, or whether it will remain a purely European affair.

There are plenty of encouraging signs that a broader international role is feasible. The British are nurturing a strategic partnership with Japan and have a thriving military trade with South Korea and Australia. Defence memorandums were inked with Vietnam and strengthened with Singapore. The network of British defence attaches in Asean nations has expanded.

And although this is unlikely to happen until 2020, Britain's newly-built aircraft carrier will also be deployed in South-east Asia. British military presence in the region will be noticeable, if not always notable.

The same applies to the French, who during the latest Shangri-La Dialogues have raised the possibility of a more permanent European presence in the region, as well as pointing out that France still has a stake, including sovereign territory in the Pacific. More significantly, Paris and London increasingly see eye to eye on China, viewing the rise of Asia's economic giant as both an economic opportunity and a strategic challenge, something few other European nations appear to grasp.

Still, it could be years before joint strategic visions end up in joint global strategies. And, at least for the moment, old instincts and old cliches prevail.

As a mark of friendship on the eve of his arrival in Britain, Mr Macron announced that his country will be lending to Britain a 900-year-old tapestry describing the conquest of England by the Normans, an event remembered by the English not as a defeat, but as the creation of their modern state. This was a noble gesture; a previous British request to borrow the tapestry for the coronation of the current British Queen in 1953 was rejected by France.

Mr Macron hoped this would remind the British of their European connections. But his gesture was immediately rebuffed by Britain's rabidly anti-European press, which swiftly nicknamed the historic item known as the Bayeux Tapestry as the "Bye-EU Tapestry".

Unsurprisingly, few French officials found this funny.