For a continent shattered by two world wars, it has always been important that Europe's leaders got on. At the start, not only language and culture but mountains of corpses divided them: if they couldn't be friends there would be no hope.
But it has never been easy. It is no accident that the three pioneers of European union, the French foreign minister Robert Schuman, the German chancellor Konrad Adenauer and the Italian premier Alcide de Gasperi, had far more in common than the vast majority of their compatriots: all three were devout Catholics from the margins of their respective countries, and all three were also native speakers of German (Schuman came from Luxembourg, De Gasperi from Tyrol).
Fast-forward to the present: never since the founding acts has the union faced such dire, divisive challenges – and again there is a winning friendship holding the whole thing together. Nicolas Sarkozy and Angela Merkel, who meet at yet another summit tomorrow, seem to be more often together than apart these days. They dine out together, they speak on the telephone; on Sunday Angela gave Nicolas a teddy bear for baby Giulia. They are capable of public spontaneity: when asked at a press conference on Sunday if they were confident that the Italian Prime Minister, Silvio Berlusconi, would succeed in passing the budget reforms he has promised, the amused glances they exchanged were worth a thousand words.
That shared smirk over the reliability of Mr Berlusconi was perhaps the truest moment of the whole event. "Merkozy" must present an image of happy togetherness or else. France and Germany have vital need of each other, and to minimise the stresses on the euro it is essential that they appear in harmony. But few people are fooled. "It's a necessary relationship," says an observer in Berlin. "In fact they don't like each other that much at all. They don't get on – the whole thing is highly stage managed."
A harsh and far from sentimental reality is always threatening to burst through the pasteboard image of Euro-togetherness. Mr Sarkozy, the man of whom one WikiLeaks cable said: "just being in the same room with him would raise anybody's stress level," is more often than not the source of it. When he yelled at David Cameron to "shut up" at the weekend, a little of the tension of being at the heart of the continent's crisis was released; and we got another glimpse of the high emotions seething away behind the scenes.
It would have been interesting to be a fly on the wall when Silvio Berlusconi met Angela Merkel again after it came out that he had described her on the telephone as "an unfuckable lard arse". One theory about that smirk she exchanged with Sarkozy was that it was very mild payback for his gross insult. But then if Europe's leaders are a family, and Merkel and Sarkozy the de facto couple at the heart of it, Berlusconi is the wayward uncle, the one who turns up drunk at Christmas and ruins the party by groping the prettiest guest.
Berlusconi had a famously appalling relationship with Sarkozy's predecessor, Jacques Chirac: although Berlusconi speaks fluent French and attended the Sorbonne in his youth, he found Chirac not only insufferably arrogant but far too tall. Sarkozy, being almost as diminutive as the Italian premier, promised better things – until he started showing what Italians regarded as typically intrusive, neo-colonial French behaviour in the Mediterranean.
Although Italy is one of the biggest eurozone economies and its policy critical to the bloc's survival, Berlusconi's peers have long given up on him, according to Mats Persson of the Open Europe think-tank. Months ago, when Italy appeared to be teetering into bankruptcy, he moved heaven and earth to pass a dramatic budget to rescue the nation's finances. But as soon as it became clear that if Italy were to fail, the whole eurozone would go down with it, the show of resolution evaporated.
"I doubt that Berlusconi has a good relationship with any of the European leaders," Mr Persson said. "It's got to the point where no one can take him seriously any more. The smirk that Merkel and Sarkozy exchanged shows where they're coming from. What he has said and done has very much strained relationships with all major European leaders."
Yet Sarkozy too showed himself capable of being notably unchivalrous on the question of Merkel's girth. "She says she's on a diet," the French President reportedly remarked to other European leaders recently, "then she helps herself to a second helping of cheese ..."
Mr Persson said: "It's never been a fantastic relationship, but they are stuck with each other. And the personality issues have to a certain extent hampered efforts to find a solution to the euro crisis, although it's true that both leaders are faced with domestic pressures as well.
"It's very difficult to quantify how important the relationship is. It's in the interest of both of them to come to agreement at the end of the day and if they had good chemistry of course it would facilitate things." Instead they are often pulling in different directions. He points out how, at the weekend, "Merkel tried to play down expectations for a comprehensive solution to the crisis," while Sarkozy insisted that "very important decisions" would be taken "in the coming days".
Relationships that might have been smoother in normal times are being put to a brutal test, Mr Persson pointed out. "The eurozone crisis has had a very divisive impact on Europe. It's made it very difficult for the leaders to maintain relationships on a happy basis." The smiles in the leaders' group portraits have never been more forced.