The United States pledged on Friday to protect “every inch of NATO territory,” following an incident in which several suspected Russian drones entered Polish airspace during a strike on Ukraine.

Tensions have escalated across Europe as leaders denounced the airspace breach on Wednesday — marking the first occurrence since Moscow’s full-scale invasion of Ukraine in February 2022 — indicating the latest escalation from the Kremlin, which has disregarded President Donald Trump’s attempt to mediate peace discussions.

Polish Prime Minister Donald Tusk stated it was “the nearest we’ve been to open conflict since World War II,” though Moscow minimized the situation, claiming it had “no intentions to target” installations in Poland.

Trump suggested on Thursday that Russia’s alleged drone intrusion into Poland might have been accidental, but U.S. acting ambassador to the United Nations, Dorothy Shea, seemed intent on reassuring Washington’s NATO partners on Friday in light of Trump’s statements.

Intruding into the airspace of a U.S. ally, “whether intentionally or not,” demonstrated “an enormous lack of respect for sincere U.S. efforts to resolve this conflict,” she stated before the U.N. Security Council.

“The United States stands firmly with our NATO allies against these troubling airspace infringements,” she affirmed. “And you can be assured, we will protect every part of NATO territory.”

Poland and the U.S. are members of NATO, which at its core is a mutual defense pact formed after World War II to address the threat posed by the Soviet Union, meaning an attack on one may be considered an attack on all.

NATO on Friday launched Operation “Eastern Sentry” to bolster the defense of Europe’s eastern flank in response to the strikes, which saw the drones shot down with the help of NATO allies.

“We must, as NATO, make clear our resolve and our ability to defend our territory, and that is exactly what Eastern Sentry is designed to do,” NATO Secretary-General Mark Rutte told a news conference at the organization’s headquarters in Brussels.

Rutte made the announcement standing beside NATO’s top commander, U.S. Air Force General Alexus Grynkewich, who said military activity would commence in the coming days, involving “a range of assets from allies including Denmark, France, United Kingdom, Germany and others.”

Rutte said NATO was still weighing the possible intent behind the alleged incursion but called the action “reckless and unacceptable,” whether intentional or not.

NATO already has substantial assets in Eastern Europe, including thousands of troops.

It did not specify how many additional troops would be involved in the new operation.

Moscow has intensified its bombing of Ukrainian cities and infrastructure since Trump and Russian President Vladimir Putin held a peace summit in Alaska nearly a month ago.

European allies have said these large-scale aerial assaults indicate that Putin has no interest in bringing the war in Ukraine to a close.