The shocking violence in Barcelona on Sunday makes the politics behind Catalonia’s separatist movement even harder to parse. Helmeted police in riot gear disrupted polling places, beat citizens attempting to vote, and left an incredible 893 people injured—all in the name of halting a referendum on Catalonian independence that national laws deemed to be illegal.

It was hard to shake the impression of a brutal, oppressive state attempting to undermine democracy. Looking on, the world saw echoes of …