U.S. President Donald Trump speaks to reporters on board Air Force One while flying in between Ireland and Washington as he returns from the World Economic Forum on Jan. 22, 2026.

Donald Trump proposed putting the world’s most powerful military alliance “to the test” in his latest social media post, which could have serious consequences.

The U.S. President has long criticized the North Atlantic Treaty Organization (NATO), which also includes 30 European allies and Canada, because he believes other members don’t contribute their fair share.

“Maybe we should have put NATO to the test: Invoked Article 5, and compelled NATO to come here and safeguard our Southern Border from further Illegal Immigrant Invasions, thereby freeing up a large number of Border Patrol Agents for other duties,” Trump said on Truth Social Thursday night.

Article 5 refers to NATO’s, which states that an “armed attack” on one member is regarded as an attack on all 32 member-states.

NATO determines on a case-by-case basis what activates Article 5—such as the “invasion by one state of the territory of another state”—but clarifies that “events lacking an international element, like purely domestic acts of terrorism, do not trigger” the mutual defense clause, even though member states may choose to assist.

In the alliance’s nearly-80-year history, the mutual defense clause has only been invoked once: during, and the U.S.’s NATO allies then supported the American response in Afghanistan, where more than 1,000 non-American NATO soldiers were ultimately killed.

Trump’s latest Truth Social post comes amid his ongoing threat to withdraw the.

“We’ve never needed them—we’ve never really asked anything of them,” the President said on the sidelines of the World Economic Forum in Switzerland on Thursday. “You know, they’ll say they sent some troops to Afghanistan or this or that, and they did: they stayed a bit back, a little off the front lines.”

The day before, Trump, in his, criticized NATO’s apparent unreliability: “I know them all very well. I’m not certain they’d be there. I know we’d be there for them. I don’t know if they’d be there for us.”

Trump specifically criticized NATO ally Denmark when he, a semi-autonomous territory, under U.S. control. He called the Nordic country “ungrateful” after falsely claiming that the U.S. “” Greenland back to Denmark after American forces defended it during World War II.

But Mark Rutte, NATO’s secretary general, pushed back to assert that NATO did assist the U.S. in Afghanistan.

“For every two Americans who paid the ultimate price,” Rutte said, “there was one soldier from another NATO country who didn’t return to his family—from the Netherlands, from Denmark, especially from other countries.”

Denmark actually had the highest per capita deaths among the military coalition members in the Afghanistan conflict: military casualty tracker lists 43 Danish soldiers killed.

“You’re not completely certain the Europeans would come to the U.S.’s rescue if you were attacked,” Rutte told Trump. “Let me tell you, they will.”