A troubling development that has largely fallen through the cracks while media and public attention is focused on Syria’s turmoil, is the revival of serious political tensions in Spain’s Catalonia region. Pro‐​independence Catalans pressed their agenda in 2017, attempting to hold a referendum on secession from Spain. In doing so, they badly overreached. The national government in Madrid barred the referendum, and Spanish security forces sent to prevent the balloting brutally attacked mostly peaceful demonstrators in Catalonia’s largest city, Barcelona. Spanish authorities then arrested the referendum’s organizers, taking some into custody while others fled the country. The pro‐​independence movement has remained largely quiescent since then.

Unfortunately, that relative calm has come to an end in dramatic fashion. Just as Catalan independence advocates miscalculated and overreached in 2017, the Madrid government appears to have done so now, imposing draconian prison sentences on the political rebels in its custody. Spain’s Supreme Court sentenced nine Catalan separatist leaders to prison terms ranging between nine and 13 years for their role in the failed independence bid. Oriol Junqueras, former vice president of Catalonia’s regional government, received the harshest sentence.

The court’s decision has triggered mass protests throughout the region. Protesters in the separatist stronghold of Girona burned tires on the train tracks, shutting down the high‐​speed rail connection between Barcelona and France. Other railways and roads were blocked at several places in the northeastern region. Angry protesters clogged Barcelona’s streets, impeding traffic and creating widespread gridlock.

Barcelona’s international airport became the focal point of protests, which disrupted operations and caused more than 100 flights to be cancelled in a single day. As thousands of protesters rallied at the airport entrance, riot police using batons charged the crowd on several occasions. Local media outlets reported that 50 people required medical attention.

The former president of Catalonia’s regional government, Carles Puigdemont, blasted the court’s decision from his exile in Belgium. He stated that the prison sentences for the separatist leaders were an “atrocity.” Puigdemont called on followers to take immediate action “for the future of our sons and daughters. For democracy. For Europe. For Catalonia.” Madrid shows no inclination to compromise, however, issuing a new arrest warrant for Puigdemont on sedition charges.

There have been troubling domestic developments in several NATO members in recent years. Greece’s economic collapse, worrisome authoritarian trends in both Hungary and Poland, and Turkey’s slide into a thinly disguised dictatorship pursuing an aggressive, maverick foreign policy are at the top of the list. Spain is now being moving up on the roster of concerns.

Madrid waged a multi‐​decade armed struggle to suppress a secessionist insurgency in the Basque region of the north. That conflict subsided in recent years and now appears to be over, with the principal armed force, ETA, disbanding in 2018. Despite the ETA’s demise, though, substantial public sentiment for independence (or at least more robust autonomy) remains strong. Tens of thousands of people participated in a pro‐​independence human chain in the Basque region in June 2018, just weeks after the ETA dissolved.

Catalans are now poised to supplant the Basques as the principal secessionist headache for Madrid. Moreover, as Puigdemont’s latest statement indicates, pro‐​independence Catalans intend to make their struggle a test of the values that NATO and European Union profess to represent, especially democracy and human rights. Writing in the pages of the Guardian, Puigdemont asserted: “The actions of the Spanish state, its government and its judiciary strike at the heart of our democratic values at the very time that Europe needs them most. This can no longer be regarded as an internal matter for Spain .…”

As I discuss at length elsewhere, NATO (as well as the EU) is showing multiple fissures regarding a growing array of political and security issues. Add Catalonia’s mounting challenge to Spain’s unity and territorial integrity to the list of potential troubles for the Alliance.