Under the current arrangement, the President must decide whether to waive sanctions on different components of the Iran deal every 90 days. In the past, he's done so.
But over the past several months, the President has signalled an interest in taking a harder line. Trump replaced several of his moderate foreign policy advisers with strident Iran deal opponents like newly confirmed Secretary of State Mike Pompeo and national security adviser John Bolton. Trump has also spent some time in consultation with Israeli Prime Minister Benjamin Netanyahu, one of the deal's most acerbic critics.
That's got experts thinking that tomorrow Trump may begin a push to more aggressively dismantle the deal.
Many predict that he'll reimpose sanctions on Iran's central bank. He may also decide whether to reimpose them on key Iranian business people and institutions. French President Emmanuel Macron says he believes Trump will ditch the deal "for his own domestic reasons". Energy markets agree, and are bracing for the worst.
In truth, though, the Iran deal is pretty complicated, and the President has several different courses of action. Here are the possibilities, ranked from deal to no deal:
1 Deal:
Trump decides not to reimpose sanctions. He announces that the US will continue to honor the 2015 accord indefinitely.
In theory, there's no reason that Trump shouldn't continue to abide by the deal. All major international bodies, including the International Atomic Energy Agency, say Iran is fulfilling its end of the bargain. The President, though, has remained unconvinced despite heavy lobbying from all European leaders.
The chances of this happening are basically zero.
2 Less deal:
Trump declines to reinstate sanctions one last time. But he promises to do so in 90 days, unless Iran makes major concessions.
If Trump decides to go this way, he would give the rest of the signatories some wiggle room to address some of the President's major grievances. Trump has criticised the deal because it has an expiration date. He doesn't like that the accord doesn't ban Iran from testing intercontinental ballistic missiles. And Trump has said that Iran needs to curtail its support of groups like Hizbollah, and stop supporting rebel groups in Syria and Yemen.
European leaders say some of these issues can be addressed. But it's hard to imagine how. Iran has already said that it won't renegotiate the deal. And some of Trump's major concerns, like Iran's support of groups like Hizbollah abroad, are beyond the scope of the accord.
"I doubt [Germany, France and the UK] could mollify Trump in a way that would stabilise the deal," Iran expert Ali Vaez writes. "Time has come for the Europeans to pivot and try to keep Iran in compliance with the deal without the US."
3 Even less deal:
Trump reinstates sanctions on Iran, but says they won't be implemented right away.
Like the option above, this gives allies some opportunity to come back to the table. But it doesn't seem like they'll be able to make any significant changes. And with this more aggressive arrangement, Iran would have even less incentive to participate.
4 No deal:
Trump decides to reimpose sanctions on Iran, and announces that the US will begin punishing those who violate them immediately.
If Trump decides to reimpose sanctions on the Iranian central bank, companies and countries will have 180 days to stop buying oil from Iran. Those who don't will be punished by the US.
It's not totally clear what might happen next. Optimists say this aggressive approach might be what it takes to get Iran back to the negotiating table. In truth, that doesn't seem likely.
It's not clear that the UK, France and Germany would play ball. Leaders from those countries have said that they'll continue to honour the deal. And Iran's ambassador to the UK told CNN last week that if the United States pulls out of the agreement, "it means that there is no deal left."
Russia may also use the opportunity to try to weaken the US by making its own deal with Iran.
5 Really, truly no deal:
Trump could impose even more sanctions than he has to.
Experts call this the "nuclear option". Trump could decide to reimpose not only sanctions on the central bank, but also sanctions on 400 Iranian people and businesses.
No matter what happens tomorrow, Trump's unwillingness to stick to the terms of the Iran deal has weakened the accord, probably fatally.
As Vaez put it, "the first option would be the best but also, based on what Macron said at the tail end of his visit, the least likely".
As for the others, he said, "they range from the undesirable to the frankly destructive. Each would, at a minimum, spell persistence of the damaging ambiguity surrounding the nuclear deal's fate".
