Iran reached a landmark agreement with world powers on Tuesday that would see it emerge from more than a decade of isolation in return for curbs on its nuclear program.

Coming after two years of negotiations with the five permanent members of the United Nations Security Council plus Germany, the deal was hailed by American president Barack Obama — who has staked much of his legacy on successful diplomacy with Iran — as one that will prevent the Middle East’s largest Shia state from obtaining nuclear weapons.

“The deal is not built on trust,” he said in a televised statement delivered from the White House. “It’s built on verification.”

Under the agreement, United Nations inspectors would be allowed to enter any Iranian facility, including military sites, where they suspect undeclared activity might be going on. But in some cases, Iran could withhold permission to inspect facilities for up to three weeks.

Once the International Atomic Energy Agency verifies that Iran is scaling back its nuclear program — which it has always denied is designed to make weapons — world powers say they will start lifting economic sanctions that have pushed Iran’s oil-based economy into recession.

Russia already unblocked the sale of missile defense systems to Iran in April when a tentative agreement was reached in Switzerland.

On Tuesday, the country agreed to reduce its number of operating centrifuges, which are used to enrich uranium, by about two-thirds and eliminate the bulk of its existing uranium stockpile. Combined, the measures should extend the “breakout” period needed to build a bomb to one year.

Iran would also suspend the development of heavy water reactors for fifteen years and convert its underground enrichment facility at Fordow — which was only discovered by Western intelligence agencies in 2009 — into a nonmilitary site.

Hassan Rouhani, the Iranian president, said the agreement “opens a new chapter” in his country’s foreign relations. “Today is an end and a beginning,” he said. “It is the end of injustice to the Iranian nation and beginning of new cooperation in the world.”

The Financial Times‘s Roula Khalaf argues that the deal is the triumph for pragmatists in Iran. Rouhani, who was elected in 2013 to end the sanctions, promised Ayatollah Ali Khamenei, the country’s supreme leader, “a nuclear deal that he can live with, even if it may not correspond fully to the dictated red lines — and he has delivered.”

If he can find a more legitimate path to Iranian ambitions of grandeur, through closer diplomatic ties with Europe in particular and a détente with the United States, he might be able to show the leader that more responsible Iranian behavior, rather than confrontation with the Sunni Gulf states, can better legitimize the Islamic republic’s role as a regional power. That is when the Middle East might begin to look different.

American allies in the region are wary of a nuclear deal for that very reason. Any rapprochement with Iran following 35 years of hostility would make the United States less dependent on especially authoritarian Sunni Arab states that are engaged in a struggle for regional hegemony with the Shia republic.

Iran was able to expand its influence in neighboring Iraq after the United States toppled the secular dictatorship of Saddam Hussein there. Since last year, it has had troops in Iraq to fight the same Islamic State militants that are subject to American airstrikes.

The former American commander in Iraq, General David Petraeus, warned in an interview with The Washington Post earlier this year that Iran’s presence in the country could aggravate sectarian tensions in the Middle East. “The more the Iranians are seen to be dominating the region, the more it is going to inflame Sunni radicalism and fuel the rise of groups like the Islamic State,” he said.

Saudi Arabia and the other monarchies in the Persian Gulf worry that Obama will accept Iran’s strategic gains in not just Iraq but places like Lebanon and Yemen as well in order to secure a nuclear deal. In both countries, Iranian proxies have taken over. The Saudis launched airstrikes against the Iranian-backed Houthis in Yemen in March to arrest what they see as Iran’s imperial ambitions.

Critics of the deal also point out that the lifting of sanctions would enable Iran to expand its support for terrorist groups.

John Boehner, the Republican leader in America’s Congress, said, “Instead of making the world less dangerous, this ‘deal’ will only embolden Iran — the world’s largest sponsor of terror — by helping stabilize and legitimize its regime as it spreads even more violence and instability in the region.”

Jeffrey Goldberg, a journalist, writes in The Atlantic, “It is hard to imagine a scenario — at least in the short term — in which Hezbollah and other terror organizations on the Iranian payroll don’t see a windfall from the agreement.”

Yet the agreement might still be a “practical necessity,” according to Goldberg.

Does this deal significantly reduce the chance that Iran could, in the foreseeable future, continue its nefarious activities under the protection of a nuclear umbrella? If the answer to this question is yes, then a deal, in theory, is worth supporting.

Israeli prime minister Benjamin Netanyahu, a longtime opponent of a deal, disagreed. “Far-reaching concessions have been made in all areas that were supposed to prevent Iran from obtaining nuclear weapons capability,” he said.

Goldberg and Politico argue that Netanyahu might find a more sympathetic ally in Obama’s successor. The candidates for the Republican Party’s 2016 presidential nomination have all been critical and former secretary of state Hillary Clinton, the likely Democratic nominee, has also sounded more hawkish on an Iran deal than her party leader. If Iran is found backpedaling or outright cheating on the agreement, a future American president could easily reimpose sanctions.