What you need to know about the Iran nuclear deal

The announcement this weekend that the United States and five other world powers had struck a deal with Iran that curtails its contentious nuclear program in exchange for limited relief from painful economic sanctions marks the most significant accord between Washington and Tehran in more than a quarter-century.

It also caps off nearly three months of whirlwind diplomacy — as swift as it was unprecedented — following a decade-long global nuclear standoff with Iran and an extended history of failed negotiations.

"Diplomacy opened up a new path toward a world that is more secure," President Barack Obama said late Saturday night from the State Dining Room in the White House just after the historic agreement was signed at the Palace of Nations in Geneva.

He applauded the interim deal as the most "significant and tangible" measure of rapprochement between the two countries since he took office.

Iranian foreign minister Mohammed Javad Zarif, for his part, said he hoped the accord would lead the way to a "restoration" of trust between Iran and the U.S., calling on all sides to treat it as an "opportunity to end an unnecessary crisis and open new horizons."

The stunning speed of the recent round of talks and subsequent deal has many observers asking: How did it happen? Here's what we know about the historic deal.

How has the United States engaged Iran in recent years?
Iran and the U.S. had severed diplomatic ties in 1979 after the Islamic Revolution and the dramatic hostage crisis at the U.S. Embassy in Tehran.

In a July 2007 televised debate with candidates for the Democratic presidential nomination, Obama famously pledged to sit down with Iranian leaders without preconditions. Hillary Clinton later called the junior senator's offer "irresponsible and frankly naive."

Two years later, President Obama and Secretary of State Clinton struggled to engage Iran after hard-liner Mahmoud Ahmadinejad, known for his inflammatory anti-American and anti-Israel rhetoric, won a much-disputed second term as president, setting off a wave of protests and a quashed democratic uprising credited with fueling the fires of the Arab Spring.

Tensions flared in subsequent years. Iran backed out of key nuclear talks in 2009. The U.S. ratcheted up crippling sanctions against Iran in 2010. Iran, which the State Department designates as the world's most active state sponsor of terrorism, boosted Syrian President Bashar Assad after the bloody civil war erupted in 2011. Meanwhile, officials at home and abroad — including Israeli President Benjamin Netanyahu — pressed the White House to take decisive steps to stamp out the Iranian nuclear program.

With the prospects of diplomacy looking more dim than ever, the Obama administration leveled "crippling" sanctions against Iran, bringing the oil industry to its knees and gutting the economy. Amid the 2012 general election, Obama promised to prevent Iran from reaching the capacity to build a nuclear bomb — but nuclear talks were stubbornly stalled.

Why did relations between Iran and the U.S. suddenly become so... warm?
Against the odds, moderate cleric Hassan Rouhani was elected president in August, sweeping into office on a platform of centrism and reform. In a September interview with NBC News' Ann Curry, Rouhani promised to drastically change his country's relationship with the world.

Rouhani "opened the way for this historic deal," Trita Parsi, the president of the National Iranian American Council and the author of "A Single Roll of the Dice: Obama's Diplomacy With Iran," told NBC News, adding that the new president was part of a coalition of officials who had "sought a compromise with the West."

"A candidate was elected running on a platform of engaging the international community and resolving the nuclear standoff," Joel Rubin, a former State Department official who now works at the non-proliferation organization Ploughshares, told NBC News. "That was the first wakeup call that something was rumbling inside Iran."

The most significant overture came in late September. With key backing from Supreme Leader Ayatollah Ali Khamenei, Rouhani arrived at the United Nations General Assembly in New York City with an eye on reconciliation. Although the two leaders didn't meet face to face during the plenary, Obama and Rouhani spoke on the phone — a dramatic move that marked the first contact after 34 years of estrangement.

"There was a new atmosphere in the air," Rubin said. "The talks probably would not have happened without that."

What led to the Geneva talks? And how did they come together so quickly?
Although Israel remained incredibly skeptical of Iran after Rouhani's post-victory charm offensive, Washington was becoming more receptive to a civil exchange with Tehran, according to Suzanne Maloney, a senior fellow with the Brookings Institution.

"The Obama administration quickly appreciated the opportunity to negotiate," Maloney told NBC News.

In fact, leading up to the U.N. meeting, the U.S. and Iran had already engaged in a series of high-level, face-to-face talks completely behind the scenes, softening the ground for the deal ultimately reached in Geneva, according to an Associated Press report published shortly after the accord was announced late Sunday.

The talks were hidden from America's tightest allies — including its Geneva negotiating partners and the Israeli government — until two months ago, which may account for how the nuclear talks appeared to begin so swiftly despite decades of intense mistrust and hostility, according to the AP.

"Privately, lots of feelers were sent out from the president's team," Rubin said. "There were a lot of signals."

Secret discussions were held in the Middle Eastern nation of Oman and elsewhere, with only a small cadre of people on the inside, the AP learned.

A handful of people — including Deputy Secretary of State William Burns; Jake Sullivan, Vice President Joe Biden's' foreign policy adviser; chief U.S. nuclear negotiator Wendy Sherman; and Oman's Sultan Qaboos bin Said al Said — acted as principal mediators and interlocutors during these back-channel exchanges with Iranian envoys and officials, the AP reported.

What was the goal of the Geneva talks?
A preliminary agreement ironed out amid the secret talks later became a template for the formal negotiations in Geneva, which convened in early November.

The official talks were aimed at reaching an accord that would put important restrictions on Iran's capacity to build a nuclear weapon. And in exchange, global powers would lift some of the painful sanctions cutting into Iran's economy.

And who were the key players?
Behind-the-scenes figures notwithstanding, the negotiating panel including the top diplomats from six world powers: the U.S., Russia, China, the United Kingdom, France and Germany — otherwise known as "P5+1," or the five permanent members of the U.N. Security Council, plus Germany.

The first round of talks ended without a deal, after France said the deal didn't go far enough — and Iranian negotiators said they had to take the proposed accord back home. But the parties agreed to resume discussions Nov. 20.

What were the major obstacles during the talks?
Sticking points included Iran’s insistence that it has a right to enrich uranium for peaceful purposes, and the tacked-on proposal that Iran stop construction of a heavy water facility in Arak. Iran has said the facility is needed to create medical isotopes for cancer treatments, but it could also make Iran more quickly capable of building a bomb.

How exactly these sticking points were resolved will greatly influence the expected political fallout in the United States and Iran. Some members of Congress are threatening new sanctions in the U.S., and hard-liners in Iran are harshly accusing Rouhani and his Foreign Minister Javad Zarif of giving up too much in the negotiations.

What are the key provisions of the deal?
According to the White House, the deal stipulates that Iran will commit to halt uranium enrichment above 5 percent and also to neutralize its stockpile of near-20 percent enriched uranium. The Islamic Republic has also committed to halt progress on its enrichment capacity. Iran will also halt work at its plutonium reactor and provide access to nuclear inspectors.

These steps, President Barack Obama said late Saturday, will "cut off Iran’s most likely paths to a bomb.”

In exchange, the United States and its allies have agreed to offer Iran "modest relief" from economic sanctions and access to a portion of the revenue that the country has been denied through these sanctions. No new sanctions will be imposed, Obama said.

What does the deal mean for Israel?
Rubin believes Israel is "rightly nervous about what it means," following years of hostile relations with Iran.

Israel and some Republicans in Congress have already called the nuclear deal hammered out between Iran and world powers very dangerous, despite assurances from the United States that the pact would make American allies in the region safer.

"What was achieved last night in Geneva is not a historic agreement. It was a historic mistake," Israel's Prime Minister Benjamin Netanyahu told a cabinet meeting Sunday morning. "Today the world become a much more dangerous place, because the most dangerous regime in the world took a significant step towards getting the most dangerous weapon in the world."

Earlier, Israel's Deputy Defense Minister Danny Danon called the agreement "extremely dangerous for the free world."

Western nations and Israel have long feared that Iran has been seeking to develop a nuclear weapons capability. Tehran denies this, saying its nuclear program is a peaceful energy project.

The White House has tried to reassure Israel that its fear that a deal would leave it vulnerable was unfounded. Late Saturday, President Barack Obama admitted huge challenges remain and said Iran's promises will be put to the test over the next six months.

"As we go forward, the resolve of the United States will remain firm, as will our commitment to our friends and allies - particularly Israel and our Gulf partners, who have good reason to be skeptical about Iran's intentions," he said.

Emphasizing the United States' commitment to Israel as well as his personal relationship with Netanyahu, Kerry said on Sunday that the two allies continue to share the same strategy and the United States will not tolerate a nuclear Iran threatening Israel.

"There is no difference whatsoever between the United States and Israel about what the end goal is,” Kerry said.

What could derail the deal?
Iran will not honor the nuclear agreement if the U.S. Congress imposes new sanctions — as some politicians have threatened — Iran’s foreign minister told NBC News after the deal was announced.

“If there are new sanctions, then there is no deal. It’s very clear. End of the deal. Because of the inability of one party to maintain their side of the bargain,” Zarif said during an exclusive interview with NBC News.

While cautious, Zarif struck an upbeat note and said all sides had to seize this opportunity to resolve the nuclear impasse.

“It was important for everybody to use the opportunity, we all knew that this was a small window that had to used, otherwise it would be shut,” Zarif added. “What I hope is important is that we will all work to a final resolution of this issue. Now we are just taking a first step, the difficult work is ahead of us.”

NBC News' Andrea Mitchell and Becky Bratu, as well as the Associated Press, contributed to this report.

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