The Iranian parliament voted to support a historic nuclear deal with the United States and five other world powers, despite attempts by hard-liners to derail it. (Ebrahim Noroozi/AP)

After many years of fitful negotiations, Iran is expected within days to start dismantling parts of its nuclear facilities and reducing its uranium stockpiles under international supervision.

Sunday is “adoption day,” the end of the beginning for the controversial landmark agreement the United States and five other world powers reached with Iran in July. If Iran satisfactorily reduces its nuclear program so that it cannot build nuclear weapons, international sanctions will be lifted.

Now, 90 days after the U.N. Security Council endorsed the deal, the Obama administration and the European Union will lay the groundwork for sanctions relief. The president will officially notify Congress of his intent to issue provisional waivers, and direct the departments of State, Treasury, Commerce and Justice to get ready. The E.U. will take comparable steps.

The public announcement that the gears for sanctions relief are in motion will signal to Iran that it can start ripping out significant parts of its nuclear infrastructure.

Among its key commitments under the deal, Iran agreed to remove thousands of uranium-
enriching centrifuges from its facilities at Natanz and Fordow and place them in monitored storage. It will pour cement into the reactor core of its plutonium processing plant at Arak, then redesign it into a less-dangerous light-water reactor. As a byproduct of the energy cycle, heavy-water reactors yield plutonium that can be enriched to weapons grade. Iran must drastically reduce its stockpile of enriched uranium. And the whole process will be monitored by inspectors from the International Atomic Energy Agency.

Richard Nephew, a former sanctions expert on the U.S. negotiating team who is now a non­proliferation scholar at Columbia University, calls adoption day “a pretty big marker” on the road to implementing the deal.

“On that date, there may not be one less centrifuge than the day before, but it starts the process when centrifuge levels will go down,” he said. “It’s the first time that’s happened since 2007, when the centrifuge numbers started reversing. From this point forward, things are going to get better.”

No sanctions will actually be eased until the IAEA verifies that Iran has completed holding up its end of the deal — a process the United States estimates could take six to nine months. Iran, eager to see sanctions lifted as quickly as possible, said it will complete the work within two months.

Tehran already has paved the way for work to begin. On Wednesday, the Guardian Council of the Constitution, which has veto power, ratified a parliamentary vote supporting the deal. In effect, that gives the green light for the Atomic Energy Organization of Iran to start the dismantling work.

Though the sanctions waivers that the Obama administration is preparing are fairly bureaucratic compared with the steps Iran must take, they make it easier for Iranian President Hassan Rouhani to live up to the deal.

Rouhani is under intense criticism from hard-line opponents of the agreement, and he cannot be seen as taking any unilateral action that would give the impression that Iran is making concessions and getting nothing in return.

The Treasury Department is expected to post a notice on its Web site cautioning that sanctions remain in place for now, but the steps Obama will take Sunday are intended to reassure foreign investors that they soon will be able to do business in Iran without getting into trouble with the U.S. government.

“The Iranians are very eager in particular to see what the Treasury Department will do,” said Barbara Slavin, a fellow at the Atlantic Council who has made multiple visits to Iran. “They are very nervous that it’s a shell game, that they won’t get the full sanctions relief they expect, and European companies in particular will be nervous investing in Iran.”

The negotiators who finalized the deal in Vienna took pains to come up with a timeline of choreographed steps that must be taken. The only open deadline is the so-called implementation day, tied to the IAEA’s verification that Iran has met its commitments. The organization says it expects to have completed by mid-December its assessment of whether Iran sought to develop nuclear weapons more than a decade ago, another required step before sanctions can be waived. The United States is convinced that Iran did have such a project, while Iran denies it ever wanted nuclear weapons and vows it will not seek to make them in the future.

But decades of enmity between Iran and the United States have created a deep vein of distrust that could derail the agreement .

“One of the problems with this deal is you have two parties who don’t trust one another,” said Robert Einhorn, who was an assistant secretary of state for nonproliferation in the Clinton administration and now is a nonproliferation fellow at the Brookings Institution.

“Each side practically expects the other to violate its obligations. At a minimum, each side expects the other to probe, to test, to nibble at the edges. And there are opponents of the deal on each side looking to pounce on any real or perceived violations by the other side. This is a highly charged, politicized environment with a lack of trust. So smooth implementation is going to be a real challenge.”

Republicans in Congress already are preparing bills that would impose more sanctions on Iran over non-nuclear issues, such as human rights violations and the powerful Islamic Revolutionary Guard Corps’ alleged support for terrorism.

“I anticipate the president will veto any legislation,” said Mark Dubowitz, executive director of the Foundation for the Defense of Democracies and a prominent critic of the Iran deal.

“But it will send a message that the next president could crack down on Revolutionary Guard activity. And it sends a message to the markets. If you’re a financial or energy company thinking of going back to Iran next year, it could be an uncomfortable place to be. You could find yourself on the wrong end of enforcement action in 2017.”

But during the coming months, the presidents of the United States and Iran appear bent on working to ensure that the agreement sticks.

“Both President Obama and President Rouhani are facing the same level of domestic challenges,” said Seyed Hossein Mousavian, a former member of the Iranian negotiating team who is a scholar at Princeton. “But they are really determined to implement the deal as agreed.”