Here's what's in the 2015 nuclear deal with Iran that Trump abandoned and Biden is vying to restore

President Joe Biden has made restoring the 2015 Iran nuclear deal a top foreign policy priority.

Reviving the deal has proved complicated, however, with both sides insisting the other make the first move. Iran has said it will not return to compliance with the deal until the US lifts sanctions, while the Biden administration has maintained there will be no sanctions relief until Tehran shows it's adhering to the pact.

Iranian and US officials traveled to Vienna in early April to participate in indirect talks on restoring the deal, communicating through European intermediaries.

The diplomats involved in the Vienna talks have seen moderate progress so far. Two working groups were established with the goal of bringing both parties back into compliance with the 2015 deal, and the talks are set to continue into next week. The State Department on Wednesday also said the US would be willing to lift sanctions on Iran that are "inconsistent" with the 2015 agreement.

The Vienna talks represent the most significant steps toward reviving the 2015 deal since former President Donald Trump pulled the US out of the agreement nearly three years ago.

The Iran deal was a crowning diplomatic achievements of former President Barack Obama's tenure and restoring it would be a major foreign policy achievement for Biden, but it has continued to be a divisive issue in Washington since it came to fruition in 2015.

Trump controversially withdrew the US from the landmark pact in May 2018.

Subsequently, the Trump administration unsuccessfully sought to squeeze Iran into negotiating a more stringent version of the deal with harsh economic sanctions as part of a "maximum pressure" campaign. Trump's approach to the issue pushed tensions between Washington and Tehran to historic heights, and raised concerns that a new war was on the horizon in the Middle East.

Trump's unilateral decision to withdraw the US from the JCPOA in May 2018 was also promptly condemned by US allies, who have scrambled to find a diplomatic solution ever since.

Before Trump withdrew from the deal, Iran's breakout time to a nuclear weapon was roughly a year. But it's now closer to a few months , according to US officials.

Critics of Trump say that his decision to abandon the pact unnecessarily sparked a global crisis and increased the prospect of war , while pushing Iran closer to developing a nuclear weapon.

Trump frequently described the deal as "terrible," and while the pact has many proponents, he is hardly alone in this view.

To understand the polarizing nature of this deal and the challenges Biden will face in vying to restore it, here's a quick breakdown of the historic pact and the debate surrounding it.

The Iran deal, explained

Former US Secretary of State John Kerry, left, meets with Iranian Foreign Minister Mohammad Javad Zarif in 2016. Frank Franklin II/AP

In July 2015, Iran and six countries reached a historic agreement called the Joint Comprehensive Plan of Action (JCPOA), popularly known as the Iran nuclear deal.

The six major powers involved in these negotiations with Iran were known as the P5+1, which stands for the United Nations security council's five permanent members (the US, France, the UK, China, and Russia) and Germany.

The deal came together after two years of intense discussions and aimed to restrict Iran's ability to develop nuclear weapons in exchange for lifting economic sanctions against Tehran.

As part of the deal, Iran agreed to reduce its number of centrifuges - tube-shaped machines that help enrich uranium - by two-thirds. It also agreed to reduce its stockpile of enriched uranium by 98% and limit uranium enrichment to 3.67%.

In other words, Tehran agreed to restrictions that would allow it to have enough enriched uranium to maintain the country's energy needs, without having the ability to build a nuclear bomb.

On top of this, Iran agreed to give access to inspectors from the International Atomic Energy Agency (IAEA), the UN's nuclear watchdog agency, to its nuclear facilities, among other facilities. Prior to Trump's decision to withdraw from the deal and for a long period after, the IAEA repeatedly found Iran to be complying with the terms of the pact.

In January 2016, when the IAEA declared Iran was living up to its end of the bargain, all nuclear-related international sanctions against Iran were lifted .

The controversy surrounding the Iran deal, explained

Iran and the US have been enemies for decades . The two countries have an extremely complex history that involved a CIA-orchestrated coup in the 1950s, a pro-American puppet monarch who was overthrown in 1979 via the Islamic revolution, and the infamous hostage crisis at the US embassy in Tehran that followed the uprising.

The constant threats from Iranian leaders against Israel, America's top ally in the Middle East, and chants of "death to America" in Iranian streets have also not helped matters.

In this context, there is a massive distrust for Iran in the US (and vice versa), and Washington has long feared what might happen if the Iranian regime developed a nuclear weapon. Iran made great strides in this regard by the 2010s, hence the Obama administration's efforts to orchestrate the nuclear deal. When the pact was finally settled in 2015, it was widely celebrated as a major diplomatic achievement.

But many (primarily conservative) leaders in Washington still felt the Iran nuclear deal didn't go far enough to limit the country's ability to develop nuclear weapons.

This is because the Iran deal contains sunset clauses , or parts of the agreement that will ultimately expire. Under the deal, the restrictions on Iran's centrifuges go away after 10 years (in 2025) and the limitations on uranium enrichment disappear five years after that (2030). Hence, some feared that once these restrictions expire, Iran could rapidly develop a nuclear weapon.

"It is clear to me that we cannot prevent an Iranian nuclear bomb under the decaying and rotten structure of the current agreement," Trump said in May 2018. "The Iran deal is defective at its core. If we do nothing we know exactly what will happen."

More broadly, Trump, among others, argued the deal didn't do enough to address Iran's regional behavior or its missile program.

The US has also faced pressure to avoid engagement with Iran from its top ally in the region. Israeli Prime Minister Benjamin Netanyahu has also been a vocal opponent of the deal, and has urged against its restoration .

Saudi Arabia, a close security partner of the US that also views Iran as a threat, has also criticized the deal and urged the US to consult the Gulf states in any efforts to revive the accord.

Iran effectively abandoned the the nuclear deal amid fears of war with the US in early 2020

An Iranian holds a photograph of Qassem Soleimani in Tehran after his assassination. Reuters

After Trump in January 2020 ordered a drone strike that killed Iran's top general, Qassem Soleimani, Tehran effectively abandoned the JCPOA altogether.

By November 2020, the UN's nuclear watchdog said Iran's stockpile of low-enriched uranium was more than 12 times the limit set under the JCPOA.

For roughly a year after Trump's controversial announcement, Iran remained in compliance with the deal. But the Iranian government began taking steps away from the JCPOA in the summer of 2019.

Wendy Sherman, who served as the Obama administration's lead negotiator on the JCPOA and has been nominated by Biden to serve as deputy to Secretary of State Antony Blinken, in September 2019 told Insider that Iran's violations of the pact were of "serious concern."

"It's all concerning, because it's moving away from a framework that ensured Iran would not get a nuclear weapon," Sherman said. "Iran is not just being emboldened but is being left in some ways to take actions that say they will not be pushed back. We are at a very, very difficult place."

Iran has gradually taken more steps in violation of the pact over the past year or so, as part of an effort to increase pressure on European leaders fighting to save the deal.

Iranian authorities in early April said that Iran had produced 55 kg of uranium enriched up to 20% - well above the limits under the 2015 deal and closer to weapons-grade levels (roughly 90%).