Poland invoked Article 4 of NATO’s treaty on Wednesday after the alliance’s fighter jets shot down Russian drones that entered its airspace in the early hours of the morning. Russian drones have crossed into Poland before, including twice last week, but this was the first time that Russian drones had been shot down over the territory of a NATO country.

“What is clear is that the violation last night is not an isolated incident,” said Mark Rutte, NATO’s secretary general. “We will closely monitor the situation along our eastern flank, our air defenses continually at the ready.”

Here’s what to know about NATO’s Article 4.

What is NATO?

NATO, the North Atlantic Treaty Organization, is a mutual-defense alliance created after World War II by the United States, Canada and 10 European countries. The alliance’s core provision, Article 5, refers to its commitment to treat an attack on one member as an attack against all of them. When NATO was created, Article 5 put Western Europe under U.S. protection, as the Soviet Union was cementing its dominance over Central and Eastern Europe.

Since NATO was founded, 20 more European states have joined: Albania, Bulgaria, Croatia, the Czech Republic, Estonia, Finland, Germany, Greece, Hungary, Latvia, Lithuania, Montenegro, North Macedonia, Poland, Turkey, Romania, Slovakia, Slovenia, Spain and Sweden. There are a total of 32 members.

What is Article 4?

Article 4 allows a member state to start a formal discussion among the alliance about threats to its security. While invoking Article 4 does not commit NATO to any military action, it is a required step toward a NATO decision to invoke Article 5. (An invocation of Article 5 is often assumed to have military implications, but the NATO treaty says only that its members will “assist” the party that has been attacked. This can also mean economic or political action.)