From Imperial Spain: 1469-1716, by J. H. Elliott (Penguin, 2002), 2nd ed., Kindle Loc. 4015-4044:
In the Netherlands, in France, the forces of international Protestantism were on the march. That it was an international conspiracy, Philip had no doubt, for each passing year showed more conclusively that the Dutch rebels were not alone. Behind them were the Huguenots, and the Breton seamen who were now waging war on Spanish shipping in the gulf of Gascony, and who were to cut Spain’s maritime communications with Flanders in the winter of 1568–9. Behind them, too, were English privateers like Sir John Hawkins, whose raid into the Spanish Caribbean in 1568 brought Spain and England a step nearer to open war.
Already by 1568 it was clear that the struggle was spreading – spreading in particular to the sea, where the Protestants were at their strongest and where Spain was still weak. The war between Spain and international Protestantism was essentially a naval war, fought in the Bay of Biscay, the English Channel, and even, increasingly, in the hitherto exclusive preserve of the Spanish Atlantic. Spain’s American possessions could no longer be regarded as safe. But for that matter it was questionable whether any part of the King’s dominions was now immune from attack. Indeed, Spain itself was threatened, both by pirate attacks on its coasts, and by armed incursions across its frontier with France.
The acute sensitivity of Philip to the dangers from heresy is suggested by his behaviour in the Principality of Catalonia. The Principality was undoubtedly one of the weaker sections of the Spanish bastions, both because of its exposed position on the French frontier, and because the extent of its privileges made it little amenable to royal control. It was well known that there were Huguenots among the bandit gangs that were constantly passing to and fro across the border, and there was every reason to suspect that heresy had found converts among that steady stream of Frenchmen which had for some years been crossing the Pyrenees into Catalonia in search of work. If heresy were to take root in Catalonia, the position would be extremely grave, since the Principality had all the makings of a second Netherlands: a strong tradition of independence, its own laws and privileges, and a hatred of Castile that was aćcentuated by linguistic and cultural differences. Consequently, as the pressure mounted against the Catalan frontier, the King’s fears grew. The viceroys were instructed to show the greatest vigilance in guarding the frontier, and in 1568 the situation appeared so alarming that severe new measures were decreed: a fresh prohibition on natives of the Crown of Aragon studying abroad; a harsher censorship in Catalonia; and a ban on all teaching by Frenchmen in Catalan schools. Then, in 1569, the Catalans refused to pay the new tax known as the excusado, which had just been authorized by Pius V. Convinced by their refusal that they were on the verge of going over to Protestantism, Philip ordered the Inquisition and the Viceroy to take action, and had the Diputats and a number of nobles arrested.
The King’s vigorous action against the Catalan authorities is an indication of his deep anxiety about the course of events. As he himself later realized, the action was unwarranted; there was no breath of heresy among the Catalan governing class. But the situation seemed sufficiently dangerous to make action essential. The Protestant peril was growing hourly, and it was growing at a moment when the danger from Islam seemed also to be mounting to a climax. For Catalonia was not the only region of Spain where revolt and heresy threatened. On Christmas night of that terrible year 1568 – the year of the danger in Catalonia, of the cutting of the sea-route through the Bay of Biscay, and of the arrest and death of Philip’s son and heir, Don Carlos, a band of Morisco outlaws lead by a certain Farax Abenfarax broke into the city of Granada, bringing with them the news that the Alpujarras had risen in revolt. Although the rebels failed to seize the city, their incursion signalized the outbreak of rebellion throughout the kingdom of Granada. Spain, which had surrounded itself with such strong defences against the advance of Protestantism, now found itself endangered from within; and the threat came not, as was expected, from the Protestants, but from its old enemies, the Moors.