IT's an old line but we all know it.

Scots, said Jim Sillars, are "90-minute patriots". Our nationalist fervour, the one-time SNP No 2 declared, barely lasts as long as a soccer match.

He could have said the same thing of the national side's next opponents, Belgium. Well, almost.

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Because the fans of the currently terrifyingly good Red Devils aren't so much "90-minute patriots" as "90-minute unionists".

Somehow people from both Flanders and Wallonia - Belgium's barely reconcilable halves - can come together to support the team for an hour and a half. And then walk away to their normal worries on pensions, debts and the fate of their bi-national state.

Take Mark Demesmaeker. He represents Flemish nationalist N-VA - allies of the SNP - in the European Parliament.

His lifelong aim is a Flemish-speaking state - and, consequently, national football team.

But he'll be rooting for the Belgium when they step out on to Hampden Park on Friday.

"My party's militants and members support the Red Devils, of course we do," he said over the phone from Brussels. "It is the only team where there are Flemish players, so we have to support them.

"I think Belgium is a special situation because we have a great division between the Flemish and French-speaking parts with different languages, different media, different public opinions and different political preferences but there is just one a national team.

"But this is a matter of sport, not politics.

"Everybody supports Red Devils but everybody realises that the reality of life will not change if they beat Scotland.

"The day after the game we'll still have the same record public debt, the highest taxes in Europe and this contrast between Flemish north and French-speaking south."

Demesmaeker stresses that the fact that Scotland has its own football team isn't lost in Flanders.

But the civic nationalist N-VA - now a real force in Flemish politics - doesn't expect to get a national side until the territory devolves itself to full autonomy.

"One day it will happen," he said. "It is the logical evolution of a state that consists of two different countries that are forced together.

"We will have our own autonomy, full autonomy, and, I hope, our own football team."

This fascinates me. Many even staunchly unionist Scots would be horrified at the idea of a Team GB in football. But fiercely nationalist Flemish fans are quite happy to support a team that is - increasingly - a symbol of the Belgian state they despise.

"The only things that are really Belgian are the Red Devils, the monarchy and the famously inefficient tax system," one Brussels insider said, repeating a classic joke. "Everything else is either Flemish or Walloon."

Big unionist guns are backing the Red Devils as the entire Belgian state begins to look like it is hanging by the shoogliest of pegs.

The manager Marc Wilmots - widely respected among both French and Flemish speakers - used to sit in parliament for a unionist party.

One of the team's stars, Manchester City's Vincent Kompany, makes no secret of his passion for his "country".

Bilingual and born in Brussels - a kind of halfway house between Flanders and Wallonia, Kompany has made overtly political statements after big games, including last year's victory over Scotland.

"He is a patriot, he loves Belgium," Wilmots said of Kompany last year. "He brings together the Flemish and the Walloons. This is national political engagement."

Will football help prop up the Belgian union? Pass.

Is Scotland's separate international footballing recognition undermining the union here? Pass again.

But Scotland's national team IS viewed with envy by those who are campaigning for independence or further autonomy in other stateless nations.

Earlier this year a Quebec sovereigntist politician, Marc Dean, began looking to Scotland for help in how to split from Canada's ice hockey team. He and other PQ leaders were, frankly, inspired by the Hampden Roar - even if it only lasted 90 minutes.

In Catalunya independentistas often point to FIFA recognition of Scotland. Some recently joined the campaign to get Gibraltar in to international football.

Wilfried Swenden, a Belgian who teaches politics and international relations at Edinburgh University, stresses his compatriots also note the curious nature of national sporting representation in the UK.

"The fact that Scotland has its own football team is quite well known and looked upon with a certain interest in Belgium," he said. "It is definitely something that has registered as part of a tradition in the UK, which predates devolution, of national sporting teams.

"People in Belgium aren't very familiar with rugby but the name of the main competition in this sport would surprise them.

"How can you talk about the Six Nations when three - and a half - of these nations are in the UK?

"Belgians are surprised by the way UK politicians can use the term 'nation'. The word is much more politically charged in Belgium, as it is in Catalunya."

Swenden, of course, has hit on one of the big differences between unionism in the UK and its counterpart movements in, say, Spain or Belgium: Britain acknowledges and embraces its multi-national identity. Up, as we reporters like to say, to a point.

The British, according to a theory well-rehearsed on the continent, have found a way, by accident or design, for small-n nationalist sentiment to be vented through sport.

After all, British unionists, the theory continues, have always tolerated "safe" expressions of national identity that didn't threaten the existence of the state. Many Scots can talk of the Scottish nation one minute and then the British one the next. That sounds odd to Belgian or Spanish ears. But will such "90-minute patriots" vote Yes in 2014? Or will they save the union?