Trump intimated that he might cut off arms sales to them for transfer to Ukraine if they reject his demand, thus likely handing Russia the victory that they’ve sought to avert for four years already, but they also don’t want to risk military losses to Iran either that could doom their politicians’ careers.
Trump warned in an interview with the Financial Times that “If there’s no response or if it’s a negative response (to his proposed Hormuz naval coalition), I think it will be very bad for the future of Nato…We have a thing called Nato. We’ve been very sweet. We didn’t have to help them with Ukraine. Ukraine is thousands of miles away from us…But we helped them. Now we’ll see if they help us. Because I’ve long said that we’ll be there for them but they won’t be there for us. And I’m not sure that they’d be there.”
The ominous innuendo is that Trump could potentially stop “helping [NATO] with Ukraine”, which could take the form of no longer selling them arms for transfer to that country, if they don’t participate in his proposed Hormuz naval coalition and “knock out some bad actors that are along the [Iranian] shore.” This throws NATO into a dilemma since its goal is to perpetuate the Ukrainian Conflict till a new anti-Russian administration comes to power in the US, but they also don’t want to risk military losses to Iran.
The conflict can’t continue if the US drops out, but the killing of troops in a distant warzone – especially a mass-casualty event like Iran sinking one of their ships – could provoke unrest and doom the careers of those who approved it during the next elections. There’s another layer to this dilemma and it’s that not helping the US reopen the strait keeps oil prices higher for longer, thus upsetting more voters, but it might also lead to the US extending its temporary sanctions waiver on Russian oil that the EU is against.
NATO therefore has to choose between helping the US secure the strait at the risk of military losses to Iran, possible unrest, and dooming the careers of those who approved it or refusing, risking the US cutting off their arms supply to Ukraine, and the US possibly extending its Russian oil sanctions waiver. The first choice entails military and political costs while the second entails economic (higher oil prices for longer) and reputational ones (worsening ties with the US and a possible Russian victory in Ukraine).
Objectively speaking, the US isn’t expected to entirely withdraw its military forces from Europe if NATO doesn’t join Trump’s proposed Hormuz naval coalition, so that dimension of the second scenario’s costs is manageable. The economic ones are too, but only if they muster the political will to discredit their own anti-Russian energy rhetoric by scaling up purchases of its oil and possibly requesting the reopening of its pipelines. The only significant cost is therefore a possible Russian victory in Ukraine.
About that, while it was earlier thought that Trump wouldn’t want to hand Putin such a victory for reasons of ego and legacy, he might do so if Putin helps him achieve some of his goals in Iran via diplomacy as was explained here, here, and here and to punish NATO for not joining his coalition. Putin could raise the odds by sweetening the terms of his proposed resource-centric strategic partnership between Russia and the US after the Ukrainian Conflict ends. This scenario therefore can’t be ruled out.
NATO should thus brace for that possibility if it rejects joining Trump’s coalition, but even if it gets involved in the Third Gulf War, then Russia could still exploit the expected redirection of Western arms to there from Ukraine to more effectively coerce Zelensky into complying with his demands. Unlike before the Third Gulf War when it looked like Putin would have to compromise on some of his demands, he now has a greater chance of achieving more of them whether by force or with Trump’s indirect support.