Twenty-one hours was not enough to end 47 years of hostility between Iran and the US.
The historic high-level talks in Islamabad, during a pause in weeks of grievous war, were always unlikely to end any other way.
Calling this marathon negotiating session a failure belies the scale of the challenge in narrowing wide gaps on complex issues ranging from age-old suspicion about Iran's nuclear programme to new challenges this war has thrown up - most of all Iran's control of the strategic Strait of Hormuz, whose closure is causing economic shocks worldwide.
To do a deal, they also needed to overcome a deep chasm of distrust.
A day ago, it wasn't even certain the two sides would meet, and even more, sit down in the same room.
A longstanding political taboo was broken.
The urgent question now is: what happens next?
What happens to the contested two-week ceasefire which pulled the world back from US President Donald Trump's alarming threat to destroy a whole civilisation in Iran?
Would the US president be ready to send his negotiators back to the bargaining table?
We're hearing reports from sources here in Islamabad that some conversations have continued after US Vice-President JD Vance boarded his plane at sunrise, declaring the US delegation had made their final and best offer.
Will the US now escalate or negotiate?
We still don't know enough about what happened behind tightly closed doors in a five-star hotel in leafy locked-down Islamabad during talks that went on long into the night.
The vice-president spoke of the core goal of the US during his brief dawn news conference.
We need to see an affirmative commitment that [Iran] will not seek a nuclear weapon and they will not seek the tools that would enable them to quickly achieve a nuclear weapon, he said.
During the last round of talks in February, before military strikes were unleashed again, Iran had offered new concessions including the dilution of its 440kg stockpile of uranium enriched to 60% - dangerously close to weapons-grade.
But it still insists on its right to enrich and hasn't been willing to give up that stockpile.
It's also refused repeated demands to open the Strait of Hormuz - to allow the free flow of vital traffic in oil, gas and other essential goods - in the absence of a new agreement.
Both the US and the Iranian delegations came to Islamabad emboldened by their belief that theirs was the winning side in this war.
And they engaged knowing that, if they failed, there was the option to keep fighting – whatever the spiraling pain for their own people and a world reeling from the cost of this conflagration.
Iran's foreign ministry spokesman Esmail Baghaei criticized the US's excessive demands and unlawful requests in a post on X.
Iran is indicating it's ready to keep talking. Pakistan's Foreign Minister Ishaq Dar urged all sides to uphold the fragile ceasefire and said they would continue their efforts to encourage dialogue.
If history provides any lessons, the last time Iran reached a nuclear deal with the US and other world powers in 2015, it took 18 months of breakthroughs and breakdowns.
Pakistani journalist Kamran Yousef described this round as one of no breakthrough but no breakdown either. The world waits for a verdict.
