On Nov. 4, Brussels Airport, one of the continent’s busiest hubs, was temporarily shut down following a reported sighting of an unidentified drone. Just days earlier, the smaller Liège Airport was also closed after drone activity, and authorities reported sightings over several military air bases, including one known to host U.S. nuclear-weapons storage.

These episodes are not isolated. Across Europe, similar drone incursions, at airports in Scandinavia, Poland, Germany and elsewhere, have raised alarm in defense and aviation circles over what analysts describe as “hybrid-warfare tactics”—unauthorized aerial systems probing critical infrastructure, and testing defenses.

These events mark an expansion in the threat landscape, as a result of the autonomous systems revolution that has been

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