What is Catalonia?
Catalonia is an autonomous region in north-east Spain with a distinct history dating back almost 1,000 years.
The wealthy region has about 7.5 million people, with their own language, parliament, flag and anthem. Catalonia also has its own police force and controls some of its public services.
Why the controversy?
For many years Catalan nationalists have complained that their region sends too much money to poorer parts of Spain.
Their budget and taxes are controlled by Madrid.
They also say Spain's changes to their autonomous status in 2010 undermined their distinctive Catalan identity.
In a referendum on 1 October, about 90% of Catalan voters had backed independence. But turnout was only 43%. Spain's Constitutional Court had declared the ballot illegal. There were clashes when Spanish national police tried to prevent people voting.
The Catalan parliament then voted for independence on 27 October. At the same time Madrid moved to impose direct control by invoking Article 155 of the constitution - a first for Spain.
What is Madrid doing?
The Spanish government has fired the Catalan leaders, dissolved parliament and called regional elections for 21 December.
Sacked Catalan President Carles Puigdemont remains defiant and has urged civil servants to disobey Madrid.
Why does the crisis matter?
The crisis does not look set to degenerate into armed conflict, but it could damage the region and Spain as a whole economically, bringing new instability to the eurozone.
It is being watched nervously by other countries with secessionist movements in Europe.
Trappings of statehood
To the casual observer, Catalonia looks like it has already got many of the trappings of a state. Flags. A parliament. Its own police force.
But there'd be much more to set up in the event of independence. Border control. Customs. Proper international relations. Defence. A central bank. Inland revenue. All of these are currently run by Madrid.
Would it be able to pay for them?
Catalonia is home to just 16% of the Spanish population, but 19% of its GDP and more than a quarter of Spain's foreign exports.
In terms of tourism - 18 million of Spain's 75 million tourists chose Catalonia as their primary destination last year.
It's also true that Catalans pay more in taxes than is spent on their region.
In 2014, Catalans paid nearly €10bn (£8.9bn) more in taxes than reached their region in public spending.
However, Catalonia's public debt is a big worry. It owes €77bn (£68bn), or 35.4% of Catalonia's GDP. Of that, €52bn is owed to the Spanish government.
Involuntary Catalexit?
Economic institutions are getting jittery over Catalonia's relationship with Europe. It would need to reapply to become a member if it seceded from Spain.
Some in the pro-independence camp feel that Catalonia could settle for single-market membership without joining the EU. Catalans may well be happy to pay for
access, and continue to accept free movement of EU citizens across the region's borders. But Spain could make life difficult for an independent Catalonia.
In practice, we just don't know what would happen.
Nobody has ever declared independence from a member of the eurozone then asked to rejoin as a new country.
Carles Puigdemont's profile
Born in Amer in 1962, Carles Puigdemont grew up under the dictatorship of Gen Francisco Franco and was taught in Spanish at a Church-run boarding school, but spoke Catalan at home like others of his generation.
As a young man, he had a passion for his native tongue, going on to study Catalan philology at the local university and polishing colleagues' copy when he first found work at the city's newspapers.
Miquel Riera worked with him at the fiercely pro-independence paper now known as El Punt Avui.
"Right from the start he was very interested in new technology and the internet," says Mr Riera. This may have fed Mr Puigdemont's awareness of social media, which was crucial in promoting the referendum campaign. Mr Puigdemont served as mayor of Girona from 2011 until 2016 when he was elected president of Catalonia.
There is no denying his star appeal among supporters, who clamour to take selfies with him at rallies and avidly follow his social media accounts, which he curates himself.
After news of the arrest warrant, he tweeted a photo of a solidarity rally outside the government office in Barcelona, writing, "This is the moment when all
the democrats unite. For Catalonia, for the liberation of the political prisoners and the Republic.”
Mr Puigdemont told Belgian TV he was not hiding from "real justice" but from the "clearly politicised" Spanish legal system.
A complex legal battle may lie ahead but by being in Brussels he is putting the cause he holds so dear, Catalan independence, squarely at the EU's doorstep.
Catalonia’s independence movement explained
Why so many people do not want their homeland to split from Spain?
Comment from The Guardian’s Simon Jenkins
Just as the EU commission in Brussels has become a parody of the pre-Reformation Roman church, so aversion to central power is destabilising states across the continent. Economies globalise and national governments centralise, and as a result regional identities become more assertive and belligerent. Not just Catalans but Scots, Basques, Corsicans, Flemings, Silesians and Venetians show no signs of diminishing their plea for more autonomy, whether heavy or “lite”. The Catalans are merely the vanguard of a movement against the clumsy bureaucratic elites that rule Brussels as they do Madrid – not to mention London. Hence it is pointless for Madrid simply to read the rule book and the riot act to solve this crisis.
It is clear that the Catalan president, Carles Puigdemont, is desperate to avoid a cataclysmic break with Madrid, knowing that his own people are deeply divided in their separatism. There has to be the sort of deal that there was with the Basques in the 1990s – perhaps even more radical. Spain is already among the most devolved states in Europe. Its recent ham-fisted treatment of Barcelona demands it become even more so, possibly with a new Catalan constitution that can be put to local plebiscite.
The EU is foolish to think this is none of its business. Its blundering diplomacy has already lost it the United Kingdom. No other nation dares hold a referendum on exit. Separation beckons from Poles, Czechs and Hungarians. The German-dominated eurozone has blighted the Greek and Spanish economies and is, in large part, to blame for the present crisis. The EU cares as little for regional identity as it does for any sort of devolution. The Catalonian question is about Europe as much as it is about Spain.