A striking current feature of the European political landscape is its convergence and separatism. As the European Union expands, secessionism keeps pace. Scotland’s referendum is next September. Artur Mas steps up the pressure on the Spanish government to timetable a poll on Catalan independence. In Belgium, borborygmal noises from Flemish separatists are a ground bass over which national politics plays.

Anyone tempted by the thought that secession is always nice has to face the history of Texas. It was part of the Mexican republic when the latter proclaimed independence in 1821. Arriviste gringos who’d been encouraged to settle there to quell banditry duly turned poacher and seceded, partly in order to keep the institution of slavery and dodge property taxes. The Texan republic acceded to the United States in 1846, once the slave-owning articles of its constitution had been ratified by the US Congress. Sixteen years later, with similar thoughts uppermost, the state of Texas joined Jefferson Davis’s secessionist confederacy in the American civil war, known to francophones as ‘la guerre de sécession’.

Seceders often want to split in order to hang on to their loot, as in Katanga, Flanders and to some degree also in Catalonia and Scotland, not to mention Ukip. That these ideas are often delusional – and, in the case of Scotland, might well condemn the rest of the UK to near-permanent Tory rule – doesn’t much increase their political attractiveness. Nor does the fact that separatists often exemplify what might be called the law of lesser crapping, with which Scottish nationalists have rightly charged Ukip: breaking away from being crapped on from above (by the EU, the UK etc) leaves the newly independent entity free to crap at will on those below (south-east England on the rest of the UK, Edinburgh on Shetland etc).

This is not to say that Catalan or Scottish secession would not be nice, only that the mere fact of secession doesn’t make it so. What legitimates secession – the separation from a sovereign territory of one of its parts, to form a sovereign entity of its own – is a knotty question of theory. How can the demos decide the limits of the demos, when that very decision, short of a global plebiscite, must be made by a limited demos? On one view, when there is question over the sovereignty of territory Z, the question should be put to a vote of the people who live in Z, and only to them. But here principle gets foxed by history. The province of Northern Ireland was created as an entity with an inbuilt (now much diminished) protestant-unionist majority: does it matter that the six counties of the historical kingdom of Ulster owe their integrity to a 1920s gerrymander? Then there are questions about enfranchising resident aliens, who pay local taxes in Z, and expat Zians who don’t. And even in a world with a simple one-to-one correspondence between citizenship and domicile, the Anschluss question comes up: when may a state be absorbed by, or remain in, another?

On the one hand, it seems daft to suggest that a country like Scotland could simply be annexed by a majoritarian plebiscite in, say, Brazil, though no doubt that would improve its World Cup prospects. On the other hand, one group’s membership in a union is something in which all members have an interest. One answer to the Anschluss question is to frame terms not just for secession, but expulsion, and indeed a sizeable minority in England, according to the latest UK social attitudes survey, favours booting out the Scots. A majority of people in England think that the Scots are getting a fair or better-than-fair deal out of the union; conversely the SNP has long maintained that Scotland stands to benefit from separation. As Northern Ireland and Texas show, colonisation can be a successful strategy in state-formation, with previous dwellers like the Comanche and old-colonial Hispanics sidelined. Since the resident English-born will get a vote in next year’s referendum, the SNP could consider bussing in pro-independence Sassenachs.

I sometimes suggest to Belgian friends that English should be the country’s one official language, and indeed at soccer internationals bladdered Walloons and Flemish unite in chanting ‘Belgium!’ rather than ‘Belgique!’ or ‘België!’ At the conclusion of Belgium’s successful World Cup qualifying campaign last week, Belgian supporters held up banners addressed to ‘Bart’ (de Wever, the Flemish separatist leader): ‘Bart, tonight you are alone.’ The banners were in English.