For a moment, history seemed to beckon. With negotiations stretching into the night and large teams of technical experts on both sides swapping drafts, there were excited rumours that, just maybe, the impossible was about to happen.
So much for that.
After 21 hours of talks at the Serena Hotel in Islamabad, JD Vance, the US vice-president, emerged bleary-eyed to call the whole thing off. He had offered the Iranians a take-it-or-leave-it deal. They left it.
“We’ve made very clear what our red lines are,” he told reporters. “They have chosen not to accept our terms.”
It did not take long for Donald Trump to respond. Within hours, he announced that the US navy would “begin the process of BLOCKADING any and all ships trying to enter, or leave, the Strait of Hormuz”.
If nothing else, the move ups the ante. By imposing additional costs on Iran’s foundering economy, Mr Trump is seeking to force the capitulation that Tehran resisted at the negotiating table.
The decision also suggests a greater sense of urgency than his earlier insouciance implied. Only hours before, he had shrugged off the failure of the talks. “We win regardless,” he said. “We defeated them militarily.” Indeed, there was an almost choreographed display of indifference to events unfolding in Pakistan.
As Mr Vance was emerging to admit defeat, Mr Trump was at a sports arena in Miami, watching a mixed martial arts fight with Marco Rubio, the US secretary of state.
It made for an incongruous split-screen. As the vice-president, a man with little negotiating pedigree, attempted to explain the collapse of the talks, Mr Rubio, America’s most senior diplomat, was watching a video montage of topless men thumping each other.
Yet Mr Trump cannot easily walk away while Iran continues to choke the world’s most important artery, through which – in normal times – a fifth of global oil supplies pass. The counter-blockade is an attempt to seize back the initiative.
It is also a gamble.
Gulf states, whose economies are already under strain, had hoped Mr Trump would use force to reopen the strait. A second blockade may not be what they had in mind. Nor are global markets likely to take kindly to a further escalation when trading opens on Monday.
There are legal risks, too. Mr Trump may find himself accused of breaching the same maritime norms he has invoked against Iran. While blockades are not illegal in wartime, their application to international straits is contested. A legal row is brewing. Nor is it clear that a blockade alone will be enough to force Iran to yield.
For all the escalation, the underlying dilemma remains unchanged. With diplomacy stalled, Washington and Tehran still face three unappealing choices: return to negotiations, resume hostilities, or allow the conflict to drift to an uneasy, unresolved end.
Mr Trump’s move suggests he still hopes to force Iran back to the table on American terms. Mr Vance’s abrupt departure from Islamabad may itself have been part of that strategy, designed to increase pressure on Tehran while leaving the door open to accept what he called a “final and best offer”.
But there is a risk that further talks would simply reproduce the current paralysis. The Obama administration took 18 months to reach a nuclear deal with Iran in 2015. Mr Trump will be wary of being drawn into another protracted process.
A return to war carries danger for both sides. The conflict is already unpopular in the United States. A second act threatens turmoil in global energy markets.
For the Iranian regime, renewed fighting risks squandering the domestic goodwill it has built by casting itself as the victim of an unprovoked war. Public sympathy may have shifted during the initial phase of the conflict, but ordinary people – many of whom have borne the brunt, with an estimated one million put out of work – may be less forgiving if a chance for peace is seen to have been squandered.
Mr Trump may calculate that the economic pressure of a naval blockade will, over time, force Iran’s leaders to bend – or even drive discontent back onto the streets. But such effects would take time, complicating his hope of declaring victory and moving on.
He could still seek to shift attention to more politically rewarding foreign policy battles – Nato, Greenland and Ukraine among them – but that becomes harder while the Strait of Hormuz remains contested.
At root, the problem is that a durable settlement remains elusive while both sides still believe they can dictate the outcome.
In Islamabad, where a chasm separated the two sides, optimists could just about glimpse a bridge. Mr Trump wants a swift resolution to the crisis. Iran needs relief for an economy already tottering before the war and now further damaged by the destruction of key industries employing hundreds of thousands.
Yet the incentives to compromise remain weak. Iranian hardliners believe they still hold the upper hand, not least because of Tehran’s grip on the Strait of Hormuz. The longer they can exert pressure on the global economy, the calculation runs, the more likely Washington is to settle for terms more favourable to Iran.