Woodrow Wilson used it to dismember the empires of old Europe; Vladimir Lenin promoted it with the aim of destroying imperialism; the UN wrote it into article 1 of its founding treaty. The right of peoples to self-determination has been a principle in international law since Versailles and confirmed as the basis for negotiations on issues as varied as Kashmir in 1948, Vietnam in 1973 and the state borders of eastern Europe in 1990.

But from Kirkuk to Barcelona, the national question has resurfaced to flummox modern democracies and confound the political tradition of technocratic centrism. As the Spanish Socialist party prepares to endorse the takeover of Catalonia by a rightwing government in Madrid, as an Italian president of the European parliament issues panicked warnings to autonomists in the northern regions of Lombardy and Veneto, those in power are having to reach for the textbooks on international law.

The issue of national self-determination is back and unitary states are struggling to cope with it. The left, particularly, seems psychologically unprepared for the eruption of struggles for democracy and social justice where nation and ethnicity, not class, is the driver. And the EU is trapped in a legal limbo. Its own founding treaty failed to include the right to self-determination of peoples – preferring instead to give that right only to nations already recognised as states, in the form of article 50.

Facebook Twitter Pinterest European parliament’s president Antonio Tajani berated those who voted for autonomy in Lombardy and Veneto. Photograph: Miguel Riopa/AFP/Getty Images

International law on this issue exists only because certain people fought for the principle of self-determination towards the close of the first world war: the subjugated peoples of the former German, Austro-Hungarian and Russian empires; Wilson, who forced the principle into the founding documents of the League of Nations, and the Bolsheviks. Having recognised the right of self-determination in theory, by the summer of 1920 Lenin realised struggles for national sovereignty had the power to tear apart the imperialist powers that had invaded Russia. In response, the Comintern ordered communist parties around the world to support “national revolutionary movements” even where they were not led by workers or the left.

One hundred years ago, then, politicians ranging from conservatives to Bolsheviks had a strong theoretical understanding of nationhood, the competing claims and the principles against which they were to be judged. This is not true today.

Timeline Eight key moments in the Catalan independence campaign Show Hide Spain’s constitutional court strikes down parts of a 2006 charter on Catalan autonomy that had originally increased the region’s fiscal and judicial powers and described it as a “nation”. The court rules that using the word “nation” has no legal value and also rejects the “preferential” use of Catalan over Spanish in municipal services. Almost two weeks later, hundreds of thousands protest on the streets of Barcelona, chanting “We are a nation! We decide!” At the height of Spain’s economic crisis, more than a million people protest in Barcelona on Catalonia’s national day, demanding independence in what will become a peaceful, annual show of strength. The pro-independence government of Artur Mas defies the Madrid government and Spain’s constitutional court by holding a symbolic vote on independence. Turnout is just 37%, but more than 80% of those who voted - 1.8 million people - vote in favour of Catalan sovereignty. Carles Puigdemont, who has replaced Mas as regional president, announces an independence referendum will be held on 1 October. Spain’s central government says it will block the referendum using all the legal and political means at its disposal. The Catalan parliament approves referendum legislation after a heated, 11-hour session that sees 52 opposition MPs walk out of the chamber in Barcelona in protest at the move. Spain’s constitutional court suspends the legislation the following day, but the Catalan government vows to press ahead with the vote. Police arrest 14 Catalan government officials suspected of organising the referendum and announce they have seized nearly 10 million ballots destined for the vote. Some 40,000 people protest against the police crackdown in Barcelona and Puigdemont accuses the Spanish government of effectively suspending regional autonomy and declaring a de facto state of emergency. Close to 900 people are injured as police attempt to stop the referendum from taking place. The Catalan government says 90% voted for independence on a turnout of 43%. Spanish government takes control of Catalonia and dissolves its parliament after secessionist Catalan MPs voted to establish an independent republic. Spanish prime minister, Mariano Rajoy, fires regional president, Carles Puigdemont, and orders regional elections to be held on 21 December.

The “principle” from which Pedro Sánchez, the Spanish socialist leader, derives opposition to Catalan independence is his own country’s constitution. But if that constitution took absolute priority, then article 1 (2) of the UN charter would be redundant. For the European parliament’s president Antonio Tajani, who berated those who voted for autonomy in Lombardy and Veneto this weekend, the principle is simply fear: fear of “the proliferation of small nations”, as Tajani put it – which, again, is not an argument in international law.

To move forward we need to understand: why are regions, states and peoples beginning to re-pose the question of national self-determination now? For Spain and Italy it is clear: the mixture of austerity, corruption and political sclerosis at the centre has limited the reality of regional democracy. It has pushed autonomous regions such as Catalonia towards independence and places such as Lombardy and Veneto towards seeking fiscal autonomy from an essentially dysfunctional central state.

Play Video 2:25 The Catalan fight for independence explained – video

But in other countries there is an action-reaction cycle under way: Britain leaves the EU; the Scottish government seeks a different form of exit, heightening the tension with the centre; republicans in Ireland spy an opening to stage the referendum on Irish unity they were promised in the 1990s, when everyone assumed economics would solve the problem.

You can see the same process happening in a different way in the French pacific department of New Caledonia, whose indigenous Kanak people were promised a referendum on full sovereignty by 2018. When I spoke to Kanak leaders in January, few believed there would be enough support for a break with France. Then in the presidential elections, the white settler population of the island swung heavily behind far-right racist Marine Le Pen, changing the dynamic.

As calls for autonomy and independence proliferate, mainstream left parties are failing to understand the basic principle: in some circumstances, the national question is not a distraction from the fight for social justice – it is the frontline of it. And it is not going away.

Above the problems of economic failure and racial polarisation, the positive factor driving progressive nationalisms, from Scotland to Catalonia, is technological change. Information-rich societies reward the development of human capital; so the ability to study in your first language, to participate in a rich national culture, to create unique local selling points for incoming foreign investment is more important than ever. If the regions, peoples and nations currently demanding more freedom seem to be driven by “cultural nationalism”, that in turn is driven by technological change plus global competition.

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The second impact of these forces is the emergence of successful big cities and devastated small towns. In large cities with dense networks of information and culture, you can survive globalisation. In small towns it is harder. So the logical economic strategy is to create a “region” or small nation focused on one big city, and develop the suburban and rural economy in synergy with that city, not the bigger unitary state. If Barcelona were not a massive global success story, the impetus behind Catalan nationalism would be smaller.

One Dublin-based financial intermediary told me that Irish businesses continually struggle to make the idea of “Ireland” salient for big Chinese investors: that is despite Guinness, James Connolly and James Joyce. How much harder must it be to project Lombardy or Veneto as a global destination for inward investment, when you’re up against a corrupt and semi-functional Italian state?

Understanding claims for secession and autonomy does not mean acceding to them: authoritative legal referendums are the method enshrined in international law to test such claims – and it is a disgrace that the EU and Spanish state have refused one in Catalonia.

But in December the European court of justice ruled that article 1 of the UN charter, which guarantees the right of self-determination to states that are not yet independent, is a legally enforceable right in the EU. It has yet to be tested in relation to Catalonia, Flanders or Scotland, but it will be.