NATO is a promise, and now it's broken.The big picture: The alliance was built on the premise that an attack on one member is an attack on all. President Trump has made that conditional: if you won't help me in my war, I might not show up for yours.NATO's mutual defensive framework doesn't actually apply in the case of Iran, a war taking place far from the alliance's territory. But it could be the death knell for the most powerful and consequential alliance of the past eight decades.Driving the news: Trump and his team have fumed at several NATO allies for denying the U.S. logistical help or access to their airspace or military bases to carry out attacks against Iran. He's called them "cowards" for refusing to join the war to reopen the Strait of Hormuz.Secretary of State Marco Rubio said the administration would now "have to reexamine the value of NATO." Trump said he might withdraw altogether.The flip side: For their part, allies have noted that Trump launched the war without their input or any international legal framework — and created the Hormuz crisis he's now demanding they resolve. Flashback: This all comes months after Trump threatened to seize Greenland, a territory of ally Denmark, and impose tariffs on any other allies who stood in his way.That was one of several increasingly existential crises for NATO that have erupted, then died down, over Trump's two terms.Until now, allies have managed to "muddle through," in part by pursuing personal relationships with Trump and via various accommodations, like agreeing to buy U.S. weapons for Ukraine when Trump refused to provide them, notes Thomas Wright of the Brookings Institution.NATO Secretary General Mark Rutte will surely try to patch things up over Iran when he visits D.C. next week.Friction point: Taken together, Greenland and now Iran have forced European leaders to confront the need for a security architecture that could stand without the American pillar.Even if they stick to their newly robust spending commitments, though, it would take several years to be able to "defend and thereby deter Russia," and perhaps a decade to fully replace the U.S., says Ivo Daalder, a former U.S. ambassador to NATO.He says NATO remains operational and stands ready to respond to any urgent threats, such as a drone incursion."The big question is, let's say there is an actual armed attack on NATO. Would there be a political decision [by Trump] to come to the aid of that ally?" wonders Daalder.Trump has given ample reason in the past, and even just this week, to suspect the answer might be no.For the allies who share a border with an expansionist Russia, that's a very worrying prospect.Zoom in: The Iran war is shaping up as a strategic windfall for Moscow, boosting oil revenues and diverting Western attention — all while straining NATO.Russian officials and state media are openly reveling in Trump's attacks on the alliance, casting them as validation of Europe's weakness and self-sabotage.Surging oil prices — coupled with Trump's "temporary" easing of sanctions — are pumping billions into Russia's war chest while highlighting the depths of Europe's energy fragility.A furious Trump reportedly threatened to stop selling weapons to Ukraine via NATO if European allies refused to help open the Strait of Hormuz, according to the Financial Times.What to watch: While Trump is once again dangling a NATO departure, a 2023 law co-sponsored by Rubio states that no president can withdraw without Congress. However, the courts could well side with Trump if he decided to test it, Daalder says.Regardless, without unyielding U.S. commitment to the Article 5 mutual defense clause, NATO has already been significantly undermined.The bottom line: When Defense Secretary Pete Hegseth was asked this week if the U.S. still stood by Article 5, he deferred to Trump, but added "you don't have much of an alliance if you have countries that are not willing to stand with you."