For those following the crisis between the US and Iran, the past few days have been bewildering. On Friday, the six-week-old ceasefire seemed doomed. Donald Trump skipped his son’s wedding to remain in the White House and was reportedly contemplating renewed military strikes on Iran. On Saturday, apprehension was replaced by optimism. Trump announced that an agreement with Iran would be concluded “shortly”. On Sunday, the US secretary of state, Marco Rubio, added to the hope by saying that there would soon be “good news”.

Iran’s leaders soon dampened the optimism. The country’s media dismissed Trump’s social media post as propaganda, and Iranian officials highlighted several remaining points of dispute. As Tehran began revealing – in very general terms – its conception of a deal, the gap between it and Washington became even more evident.

The Trump administration wants an immediate re-opening of the strait of Hormuz. It also insists on the removal of Iran’s entire stockpile of enriched uranium and a ban on enrichment. The Iranians, however, want to proceed in two phases. In phase one, the ceasefire, which would include Lebanon, would be extended for 60 days. The strait would be opened without ships being required to pay tolls, and the US would lift its naval blockade and begin unfreezing Iranian assets and lifting economic sanctions.

Even this first phase could fail. Israel, alarmed by the deal, wants to preserve its freedom of action in Lebanon. And it’s hard to fathom how Washington’s insistence on a free and open strait can be squared with Iran’s 18 May unveiling of a Persian Gulf Strait Authority that will oversee maritime traffic and levy tolls on shipping.

The US and Israel have also insisted that Iran must reduce the number and range of its ballistic missiles and cease support for its regional partners, Hezbollah, Hamas and Ansar Allah (generally referred to as the Houthis) in Yemen. Iran has not mentioned either point, but Israel and its US advocates will surely object if Trump gives ground.

Assuming the strait reopens, Iranian ports are unblocked and the US begins easing sanctions and releasing frozen assets, Washington and Tehran will move to the next phase, which will focus on Iran’s nuclear programme. Yet Tehran hasn’t specified what it is prepared to do and won’t do – not a minor matter given that Trump has stated more than 70 times that Iran cannot be allowed to have nuclear weapons.

The US has demanded that Iran’s entire stockpile of enriched uranium – not just the approximately 450kg of uranium that has been enriched to 60% – be transported out of the country, something its supreme leader, Mojtaba Khamenei, has apparently rejected. Downblending the uranium under international monitors is one way around this problem, but it may not be acceptable to Trump, who is already under fire from hawks for making concessions to Tehran. If the US and Iran fail to reach agreement on the nuclear question, they could be back at square one. That, in turn, will endanger the ceasefire.

Optimists can interpret Iran’s position as an opening bid, parts of which will be sacrificed for concessions from the US. Tehran wants war reparations but must know that Trump will never agree to pay them. It could drop that demand in exchange for, say, faster sanctions relief. Optimists recognise that Trump is under pressure – the price of oil and other commodities has surged because of the war, which a majority of Americans oppose – but, as they see it, Iran also needs a deal; the US blockade has increased the price of food and medicine and forced business closures.

But as pessimists see it, the ceasefire could unravel if Trump concludes during phase two that he must resume the war to gain more leverage, or if Israel attacks Hezbollah – or even Iran. They will also note that the announcement of the deal’s details – which Trump said would happen soon – has now been delayed because of persistent disagreements.

Even if all these obstacles are overcome and a final agreement emerges, the terms will favour Iran more than the US. True, Trump could resume military strikes to extract more concessions from Iran, but there is no guarantee that the use of force would be any more successful than it was between 28 February and 8 April. Plus, renewed war would damage the US and global economy further, something Trump wants to avoid as the midterm elections draw closer.

Assuming that Trump doesn’t back away from the deal under mounting pressure from its opponents, he will be lucky to get terms similar to the Obama administration’s 2015 nuclear deal with Iran. That won’t be much of an achievement considering that he has spent $29bn as of mid-May on a failed war that has roiled the global economy.

• Rajan Menon is a professor emeritus of international relations at the City College of New York and a senior research scholar at Columbia University’s Saltzman Institute of War and Peace Studies. His co-author, Daniel R DePetris, is a fellow at Defense Priorities and a syndicated foreign affairs columnist at the Chicago Tribune