(This post was last updated at 9:18 a.m. ET.)

Years of delicate diplomacy and decades of geopolitical animosity gave way to a historic agreement Tuesday between six world powers and Iran over the country's nuclear program.

After a final, marathon negotiating session that was preceded by a series of deadline extensions over the course of three weeks, foreign ministers from the U.S., Iran, Russia, China, U.K., France and Germany gave word that they had reached an accord.

"This is a historic moment," Iranian Foreign Minister Mohammad Javad Zarif said Tuesday morning. "We are reaching an agreement that is not perfect for anybody, but it is what we could accomplish, and it is an important achievement for all of us. Today could have been the end of hope on this issue. But now we are starting a new chapter of hope."

Calling on Congress to support the deal, President Obama said the agreement cuts off all the pathways Iran has toward a nuclear weapon. He said the alternative to this deal would leave Iran closer to becoming a nuclear power and would make a military confrontation with Iran more likely.

"We give up nothing by testing whether or not this problem can be solved peacefully," Obama said.

According to the outline of the deal put out by the Obama administration, the terms would still allow Iran to enrich uranium, but only to 3.67 percent, which is needed for civilian purposes but much lower than would needed for a weapon. The deal also commits Iran to cut down on the number and types of centrifuges it can use, would set up a comprehensive inspections regime and limit the nuclear capabilities of Iran's most controversial nuclear facilities: the underground bunker Fordow and the Arak heavy-water reactor.

Secretary of State John Kerry said the bottom line is that the deal would increase Iran's so-called breakout time — or the time it could take Iran to make enough material for a nuclear bomb. According to Kerry, once the agreement is implemented, Iran's breakout time goes from two to three months to one year or more.

In exchange, the United Nations and Western powers would drop sanctions in phases, giving Iran an infusion of capital and, more importantly, allowing it to rejoin international financial systems and sell more oil.

Announcing the deal, Federica Mogherini, the High Representative of the European Union for Foreign Affairs and Security Policy, said the deal meant Iran accepted that its nuclear program would remain "exclusively peaceful" and would mark a "fundamental shift" in the way the country undertakes its nuclear program.

"We delivered on what the world was hoping for: a shared commitment for peace," she said.

She added: "What we are announcing today is not only a deal; it's a good deal."

In a lot of ways, however, this is far from over: The agreement still has to be approved by various world capitals.

In Washington, where Congress has two months to review the deal, it has been unpopular among Republicans. (If you remember, despite protestations from the White House, Republican Speaker John Boehner invited Israeli Prime Minister Benjamin Netanyahu to speak to Congress against the deal.)

Just hours after the deal was announced, Boehner issued a scathing rebuke of the deal, signaling the tough legislative fight ahead.

Boehner said the deal outlined today fell short because it did not completely dismantle Iran's nuclear capabilities. He said instead of curbing Iran's nuclear ambitions, the deal was "likely to fuel a nuclear arms race around the world."

"We will fight a bad deal that is wrong for our national security and wrong for our country," Boehner said.

Since the Iranian Revolution in 1979, the nuclear issue has alienated Iran and vexed Western powers. But reaching a sweeping solution has been a foreign policy priority for President Obama.

Beginning with secret talks at the end of 2009, the U.S. and Iran laid the groundwork for an agreement. With a phone call to Iranian President Hassan Rouhani in September 2013, Obama became the first American president to speak directly to an Iranian head of state since 1979.

That phone call opened the door for an interim deal, which was reached in November 2013. The P5+1 (the five permanent members of the United Nations Security Council — U.S., Russia, China, U.K., France — plus Germany) and Iran then reached a framework accord in April. After fits and starts and disagreements over issues that were touched on in that accord, diplomats emerged in Vienna late today saying they were ready to make history.

NPR's Peter Kenyon tells us that it is still unclear what it would mean if the deal were to be rejected by one or more of the parties.

"They could try to set a date for a renewed effort, but as a practical matter, non-proliferation experts say, it would kill the momentum and likely lead to a longer setback," Peter told us. "In 2005, it led to several years of buildup, hostility and sanctions. But there isn't a rulebook for that eventuality."

It also depends on who rejects it. If it's the U.S. or Iran, "it probably means no quick return to the table," Peter says.

Copyright 2021 NPR. To see more, visit https://www.npr.org.