President Trump can't abandon NATO on his own, but he can still turn the relationship toxic as he rails at American allies who sat out his Iran war.The big picture: America has been the backbone of the transatlantic alliance since 1949, but experts warn Trump's rhetoric and behavior threaten to undermine NATO's mutual trust even if the U.S. remains a member."It's already pretty well understood and feared amongst a number of Europeans that even if the U.S. stays in NATO, it's unreliable," Mark Webber, an international politics professor at the University of Birmingham, tells Axios.The latest: On Thursday, Trump wrote that "None of these people, including our own, very disappointing, NATO, understood anything unless they have pressure placed upon them!!!" following a closed door meeting with Secretary General Mark Rutte.Trump has repeatedly castigated European allies for missing their NATO defense spending obligations.He's recently upbraided them for not supporting the Iran war (though some provided logistical support) or for placing restrictions on America's ability to launch attacks from their territory.America's allies have noted that Trump launched the war without their input.What they're saying: "As President Trump said yesterday, NATO was tested, and they failed," a White House official said in a statement to Axios.As angry as he is, Trump can't withdraw from NATO without Congress under a 2023 law.But former NATO ambassador Ivo Daalder says Trump has stirred doubts about whether the U.S. will fulfill its collective defense commitment under Article 5, a pillar Trump has undercut.Eroding that trust, Daalder warns, is "going to linger" beyond the Trump era, pointing to recent polling showing GOP favorability toward NATO membership cooling. At the same time, he notes, Europe is doing more to ease its reliance on the U.S. military.Zoom out: Short of withdrawing, Trump could punish allies by pulling U.S. officers from NATO's command or spoiling diplomatic progress by withholding consensus — the process the alliance uses to make decisions.The Wall Street Journal reports the White House is considering a plan to move troops out of countries deemed unhelpful. The plan reportedly could involve closing a base in Europe.Yes, but: "There is not a punishment that we're going to give NATO or an ally that doesn't hurt us too," says Jim Townsend, a former deputy assistant secretary of Defense for European and NATO policy.Moving bases or troops impacts defense readiness. It could also be expensive, requiring new infrastructure, schools and housing."From a financial perspective, it makes no sense," Daalder says. "From a military perspective, it makes no sense."Reality check: Webber tells Axios he remains "reasonably sanguine" about NATO's future, noting day-to-day work is still happening "beyond the political line of vision of Washington."Historically, he says, the U.S. has led the construction of consensus, even if it's now being done with less "enthusiasm" and emphasis on topics the administration dislikes, such as climate change. What we're watching: The political crisis within NATO could always deepen. Trump on Wednesday teased the U.S. military's "next Conquest" after Iran.Trump has repeatedly mused about seizing Greenland, a territory of NATO member Denmark.The reaction from Europe, Daalder says, "would be even even more vociferous if that were to happen than it was in January."Go deeper: NATO slides into Trump-induced coma