When Antony Blinken, the US secretary of state, told reporters in New York on Friday that the coming days will determine the future path of the Middle East, he could not have been more prescient, even if at the time he was hoping that Hezbollah and Israel could be persuaded to step back from the brink.

Now, with the Hezbollah leader Hassan Nasrallah confirmed killed, the region, after 11 months, has finally stepped over the brink and into a place it has truly never been before.

All eyes will turn to the response by Tehran. It faces the fateful choice it has always sought to avoid and one its new reformist leadership in particular did not wish to make.

If it simply angrily condemns Israel for the destruction of the centrepiece of the axis of resistance that it has laboriously built up over so many years, or calls on others to take unspecified action, Iran’s credibility is in jeopardy.

But pragmatism may lead Iran to advise Hezbollah to absorb the losses and accept a ceasefire that does not also bring about a ceasefire in Gaza, Hezbollah’s stated objective.

If on the other hand Iran instead launches a direct military reprisal against Israel, it has to be meaningful. It knows it will be going into battle against a military that has proved the deadly value of its vastly superior technological and intelligence capabilities. Israel’s intelligence has clearly penetrated deep inside Hezbollah and may have done the same in Tehran.

For the new president, Masoud Pezeshkian, elected on a ticket of lifting economic sanctions partly by building better relations with the west, Nasrallah’s death could not come at a worse time.

His foreign minister, Sayeed Abbas Araghchi, had just spent a full week in New York on the sidelines of the UN general assembly, meeting European politicians such as Germany’s foreign minister, Annalena Baerbock and the British foreign secretary, David Lammy, in an attempt to persuade them to reopen talks to restore the nuclear deal that was sealed in 2015 – and Donald Trump tore up in 2018.

Rafael Grossi, the head of the UN nuclear inspectorate, had been impressed by what he heard from the meetings, saying: “I think this is the moment when it is possible to do something about the nuclear issue. The advantage of Mr Araghchi is that he knows everything about this process so he allows it to move faster”. Nasrallah’s killing makes it that much harder for the reformists to persuade the Iranian military that an olive branch still makes any sense.

Pezeshkian had already been complaining that he had received little in return for listening to western-inspired pleas not to seek immediate revenge for the killing of Ismail Haniyeh, the Hamas leader assassinated by Israel in Tehran.

Pezeshkian said he had been promised that a Gaza ceasefire deal that would see the release of hostages and Palestinian political prisoners was only a week or two away. The deal never materialised because, in Iran’s eyes, the US refused to put the pressure required on Benjamin Netanyahu to accept the ceasefire terms.

Let down once, Pezeshkian is hardly inclined to believe US vows that it had no prior knowledge of the plan to kill Nasrallah – and, anyway, Netanyahu might have sanctioned his death from a hotel bedroom in New York, but it was US-supplied bombs that exploded in Beirut.

In what is likely to be a holding statement, Iran’s supreme leader, Ayatollah Ali Khamenei, called on Muslims on Saturday “to stand by the people of Lebanon and the proud Hezbollah with whatever means they have and assist them in confronting the … wicked regime [of Israel]”.

For Washington, this is a diplomatic humiliation and a display of its inability, or refusal, to control its troublesome ally.

Netanyahu hopes to have played American diplomats for fools in New York. The US state department insists it had a clear understanding on the basis of conversations with Ron Dermer, Israel’s strategic affairs minister, and Netanyahu that Israel would accept a 21-day ceasefire, and yet as soon as the plan was announced, Netanyahu reneged on the deal.

In some ways, it is the culmination of nearly 12 months of an American strategy that now lies in ruins. Time after time since the 7 October attacks by Hamas, the US has asked Israel to adopt a different strategy over the delivery of food into Gaza, protection zones, a ground offensive in Rafah, the terms of a ceasefire and, above all, over avoiding conflict escalation.

Each time, Netanyahu acknowledged the US position, sidestepped a clear response and then ultimately ignored Washington. Each time, the US – vexed and frustrated – has expressed misgivings about Netanyahu’s strategy, but each time it has continued to pass the ammunition.

With a presidential election near and Netanyahu enjoying a surge in domestic popularity – as well as few Arab states shedding tears about Nasrallah’s demise – the US appears to have few options available. Netanyahu insists he is winning and on course for total victory.

At the moment, unless Iran proves to be more decisive than it has been so far, it is Netanyahu the great survivor who is in the driving seat.