The Spanish government and the King of Spain were shouted down ("Out!") and whistled out when they arrived to stand at the head of the anti-terrorism march in Barcelona on Saturday, organised, in theory, in memory of the victims of the recent jihadi attacks in Catalonia and as a "unitary" stand against murderous evil.

While aerial footage of the long tail of what local police estimated to be some 500,000 marchers showed few flags, the area behind the authorities—the area that received the most television attention—was a sea of Catalan separatist ensigns.

And while the main front bann...