On Monday morning a plant belonging to WB Electronics, one of Poland's largest military equipment manufacturers, burned down. The factory makes reconnaissance drones and loitering munitions - FlyEye and Warmate - precisely the ones Ukraine uses against Russian forces. The fire was put out quickly and nobody was hurt.

Polish prime minister Donald Tusk did not try to wrap it in diplomatic cotton wool. He said the fire was set deliberately and that Russia is most likely behind it. "This is a continuation of Russian escalatory activity across the whole region," he said, then added a sentence worth reading twice: "Unfortunately, this is not the first such case, and worse still, it certainly will not be the last."

Polish prosecutors classified the case as a terrorist act carried out by a foreign intelligence service, with the fire deliberately set. That is not a phrase anyone writes lightly - it is a NATO member state officially saying that a foreign intelligence service is operating on its territory.

Not one fire, but a series

On Sunday, JFG Composites in Lublin also burned, a firm producing composite parts for aircraft. In August in eastern Germany, near Leipzig-Halle airport, three drones carrying explosives were found - one of them close to a Ukrainian Antonov transport aircraft.

The government of German chancellor Friedrich Merz blamed Russia for the drones and announced measures. Brussels backed Germany and called for stricter visa restrictions on Russian nationals.

Where the "hybrid" stops

This is the grey zone that has been written about for years. It is not war, because there are no tanks. But it is enough for a factory to burn in a country covered by NATO's Article 5 without anyone calling for consultations. Where is the line? Who draws it, and what has to happen for it to be crossed?

The Balkans know this scene well. Things happen, but nobody calls them by their real name, because the name brings obligations with it. The difference is that here we are talking about factories supplying Ukraine, not about a political quarrel.

Tusk has already said what others avoid: the next fire is not a question of whether, but of when.