Poland has triggered Article 4 of the North Atlantic Treaty Organization, NATO, after the country reported 19 “breaches” of its airspace by Russian drones on Wednesday.

A senior U.S. Marine Corps official told lawmakers he believes that Russia’s drone incursion into Polish airspace earlier this week was most likely intentional. Gen. Christopher Mahoney, the current Assistant Commandant of the Marine Corps and President Donald Trump’s nominee to be the next vice chairman of the Joint Chiefs of Staff,

Multiple Russian drones crossed into Poland in what European officials described as a deliberate provocation, causing NATO to send fighter jets to shoot them down.

NATO fighter jets shot down multiple Russian drones that violated Polish airspace during an attack on neighboring Ukraine early on Wednesday, as the military alliance denounced Moscow for “absolutely dangerous” behavior that ratcheted up tensions to a new level.

NATO’s supreme commander in Europe, U.S. Gen. Alexus Grynkewich, said Thursday that the alliance wasn’t certain about the number of drones that entered Polish territory, nor whether they were fired intentionally.

The incident represented the most serious spillover of the war in Ukraine for NATO, as its aircraft engaged the drones that it said came from Russia.

Five of the Russian drones downed in Polish airspace could have been targeting a Nato military base, according to reports.

This situation brings us the closest we have been to open conflict since World War II," Polish Prime Minister Donald Tusk said.

The incursion was the first time alliance planes had engaged enemy targets in NATO airspace. Poland’s leader called it “a large-scale provocation.”

Multiple Russian drones crossed into Poland in what European officials described Wednesday as a deliberate provocation, causing NATO to send fighter jets to shoot them down. A NATO spokesman said it was the first time the alliance confronted a potential threat in its airspace.

WARSAW, Poland (AP) — Polish Prime Minister Donald Tusk pledged Thursday to push ahead with a “great modernization program” for his country’s military, a day after Russian drones crossed into Poland and amplified international tensions around Russia’s invasion of Ukraine, including what the Kremlin’s future territorial ambitions might be.