Landmark agreement or historic mistake? That’s the debate swirling around the nuclear deal Iran signed in July with the United States and five other nations.

The agreement, which took 20 months of tense negotiations, is intended to curtail Iran’s ability to develop a nuclear weapon for more than a decade. In exchange, Iran will get some relief from international economic sanctions that have crippled its economy.   

Supporters of the deal, including President Obama, say it’s the best hope for preventing Iran from building a nuclear weapon. Without it, they argue, a military confrontation with Iran is almost inevitable.

“The choice we face is ultimately between diplomacy and some form of war—maybe not tomorrow, maybe not three months from now, but soon,” Obama said. “How can we in good conscience justify war before we’ve tested a diplomatic agreement that achieves our objectives?”
But critics, including some members of Congress and U.S. allies in the Middle East, say Iran’s leaders can’t be trusted.

“It’s going to hand a dangerous regime billions of dollars in sanctions relief while paving the way for a nuclear Iran,” said Speaker of the House John Boehner, a Republican.

The agreement—between Iran and the U.S., the United Kingdom, France, Germany, Russia, and China—puts restrictions on the amount of nuclear fuel Iran can keep for the next 15 years. The current stockpile of low-enriched uranium will be reduced by 98 percent, most likely by shipping much of it to Russia. Iran’s centrifuges—fast-spinning machines that process uranium into a weapons-grade form—will be reduced by two-thirds.

Those limits are intended to ensure that it would take Iran a year to make enough nuclear material for a single bomb if it decided to abandon the deal. Analysts say that Iran—which has long maintained its nuclear program is only for peaceful purposes like generating electricity—could currently make a weapon in two to three months if it chose to do so.

Critics say the deal is bad for several reasons: The 24-day warning before inspections will let Iran hide illicit activities; there are too many loopholes; and the deal only postpones, rather than eliminates, Iran’s ability to develop a nuclear weapon and threaten U.S. allies like Saudi Arabia and Israel, which Iran’s leaders have said should be “wiped off the map.”

Israel’s prime minister, Benjamin Netanyahu, called the deal a “historic mistake” that would create a “terrorist nuclear superpower.”

Congress had a deadline of mid-September to weigh in on the deal. While most Republicans and some Democrats were planning to vote against it, most political analysts didn’t think there were enough votes—the required two-thirds majorities of both the House and Senate—to override President Obama’s certain veto of any vote against the accord.

If the deal goes through, could it lead to improved ties between Iran and the U.S.? For that to happen, the two countries would have to overcome a long history of deep suspicions on both sides.

“The deal isn’t going to solve all of America’s problems with Iran,” says Gary Sick, an Iran expert at Columbia University in New York. “But there’s a very real chance that we’ll now be able to talk to each other behind the scenes about issues that matter. It’s a watershed moment.”

It’s been 36 years since Iran’s 1979 revolution, in which radical Shiite Muslims overthrew Iran’s monarchy and imposed strict Islamic rule on what had been a Western-leaning country and close U.S. ally. That same year, a group of militants seized the U.S. embassy in Tehran, holding 52 Americans hostage for 444 days. 

The U.S. and Iran have been at odds ever since, and Iran’s defiance of the international community over its nuclear program in the last decade increased the tension. Iran has done other things to anger the West. It has armed groups like Lebanon’s Hezbollah, which the U.S. considers a terrorist organization, and backed Syria’s dictatorial president, Bashar al-Assad, in his nation’s civil war.

Though the hardline Islamic clerics who run Iran are still hostile to the U.S., many Iranians are eager to improve relations. And young Iranians are increasingly demanding more freedoms from their government. They were instrumental in electing Hassan Rouhani—a moderate who supported negotiations with the West—to the presidency in 2013.

Tired of years of devastating economic sanctions and international isolation, many Iranians—especially young people—celebrated the nuclear deal.

“I was so happy when I heard about the nuclear agreement because I think that life will be better in Iran,” Nasim, a 32-year-old business consultant in Tehran, told The Telegraph, a British newspaper. “It will be good for all the young people.”