Forget Brexit. The referendum that really matter is this Sunday in the wealthy Spanish province of Catalonia and its great city of Barcelona. A Catalan vote for independence from Spain would trigger a similar vote in Euskal Herria, the Basque country, and start Spain down the road to disintegration. The outcome of the referendum is considered too close to call, which is why the Madrid government has declared it illegal and done everything in its power to suppress it.

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Anyone who thinks Balkanisation was a 19th-century phenomenon is a fool. Earlier this month the Kurds, the world’s largest “people without a state”, voted to secede from Iraq, not to mention the Kurdish regions of Iran and Turkey. War beckons. Secession in Ukraine has produced two de facto states. Yemen is fighting itself. Myanmar is seeing the enforced secession of the Rohingya, whom a majority of Burmese do not want living in their land. In Britain, Brexit itself is a form of secession.

When sovereign states see their power eroding, they act irrationally. The Spanish government’s attempts to suppress Sunday’s vote, with police raids, media censorship and imported riot police, could hardly be more counter-productive. It was London’s inept denial of devolution to Scotland in the 1980s that stirred Scottish nationalism. It was in large part the breakneck pace of the EU’s build-up of power that drove Britain to the Brexit door.

The trouble is that democracy as “the will of the people” does not say which people. Britain defended the wish of the Falklanders to stay British, but not the citizens of Hong Kong or Diego Garcia. Britain went to war for Bosnia and Kosovo to secede from Yugoslavia and Serbia respectively. It would be happy if Northern Ireland voted to secede and re-join the Republic of Ireland, yet it fanatically opposed Scottish independence. Secession is a concept riddled with double standards.

The only sensible conclusion is to acknowledge the right of territorial groups to some form of self-rule. The liberty of a democratic state to impede its own break-up is qualified by the right of its provinces to decide for themselves how they want to be governed and by whom. The peaceful partition of Czechoslovakia was a case in point. Where agreement is not reached, secession is the default. The alternative is conflict and, in the extreme, violence.

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That is why the issue is not secession as such, but the state centralism that is usually its cause. The conflict between Madrid and the Basques led eventually to compromise and “autonomy-lite”. This must surely be the sensible outcome of the Catalan dispute. How superior power treats inferior rights has lain at the root of politics since the dawn of time. Devolution is not an option but a necessity.

• Simon Jenkins is a Guardian columnist