Russian drones breached Poland’s airspace and, experts said, by Kremlin design or not, the incident tested NATO’s readiness to defend its easternmost members.

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.

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.

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,

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.

Russian drones shot down over Poland were reportedly headed for Rzeszow Airport, a key logistics hub for aid to Ukraine, German media outlets Spiegel and Welt reported on Sept. 11.