Copenhagen

When Europe’s leaders touched down in Copenhagen this week, they did so under the shadow of a new kind of threat. Airports across Denmark had only just resumed normal operations after unexplained drone activity disrupted flights and rattled the country’s airspace. The timing could not have been sharper: within days, dozens of presidents and prime ministers would gather for back-to-back summits on security and cooperation.

Denmark responded with extraordinary measures. Civilian drone flights were banned, air traffic was heavily restricted, and foreign troops quietly deployed radar and counter-drone systems. A German frigate even dropped anchor in Copenhagen’s harbor, a reminder …