Poland accused Russian forces of violating its airspace with hostile drones overnight in the course of conducting attacks on Ukrainian targets, and it activated Article 4 of NATO’s treaty.

The incident has further raised tensions between Russia and the NATO security alliance, of which Poland is a member.

Here’s what to know.

The facts

Polish officials said they detected 19 Russian violations of the country’s airspace between 11:30 p.m. Tuesday and 6:30 a.m. Wednesday, adding that it was the first time drones entered Polish territory directly from Belarus. There were no reports of casualties.

Poland’s military said it shot down at least three of the Russian drones, with the help of Dutch F-35 jets that are part of NATO’s allied air command. NATO said it was the first time its aircraft

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