Iran and six world powers reached a landmark nuclear agreement Tuesday after more than a decade of negotiations, striking a compromise over U.N. inspections of Iranian military sites in return for the gradual lifting of sanctions.

“We delivered on what the world was hoping for: a shared commitment to peace and a joint effort to make the world a safer place,” European Union foreign policy chief Federica Mogherini told reporters, adding that Iran had promised that “under no circumstances” would it try to obtain or make nuclear weapons.

Concluding 18 days of talks in Vienna between Iran and the P5+1 group — the U.S., U.K., France, China and Russia plus Germany — she and Iranian Foreign Minister Javad Zarif both hailed the deal as the opportunity for a “new chapter” in relations with the Islamist republic.

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“Every path to nuclear weapons is cut off,” U.S. President Barack Obama said in Washington, reassuring skeptics that the deal is “not built on trust” but on verification, with international inspectors assured access “where necessary, when necessary.”

“If Iran violates the deal, all these sanctions will snap back into place,” said Obama.

Israel quickly condemned the deal, as did as some Sunni Arab monarchs, and it now faces review by a hostile Republican-led U.S. Congress. However, a successful outcome could boost Obama’s checkered foreign policy record and be the crowning achievement of Secretary of State John Kerry’s 30-year political career.

Tehran and the six powers — who want Iran to scale back its sensitive nuclear activities to ensure that it cannot build a nuclear weapon — will now agree a “road map” setting out the technical details of how to implement the deal by December, said Yukiya Amano, director general of the International Atomic Energy Agency.

Iran’s Zarif described the deal as a “win-win” solution — though not perfect. Hassan Rouhani, Iran’s president, tweeted: “If #IranDeal, victory of diplomacy and mutual respect over outdated paradigm of exclusion and coercion. And this will be good beginning.”

“We are reaching an agreement that is not perfect for anybody but is what we could accomplish. Today could have been the end of hope, but now we are starting a new chapter of hope,” said Zarif.

“This is a sign of hope for the entire world,” said a beaming Mogherini.

Diplomatic sources said the deal includes a compromise that would allow U.N. inspectors to press for visits to Iranian military sites as part of their monitoring duties, though such access would not automatically be granted. Tehran would have the right to challenge any U.N. request and an arbitration board would have to decide.

Iran, which wants international sanctions lifted, has always insisted that its nuclear work is peaceful.

Israeli Prime Minister Benjamin Netanyahu described the agreement as “a bad mistake of historic proportions.”

“Iran is going to receive a sure path to nuclear weapons,” he said.

Breakout time

Although a long and complex implementation period leaves it vulnerable to unraveling, the deal could prove one of the modern era’s most important arms control agreements, in a league with the 1970 international Nuclear Non-Proliferation Treaty and the START nuclear missile treaty that the U.S. signed with the Soviet Union in 1994.

Obama has made the Iran nuclear talks a central plank of his foreign policy. Dating from early in the 2008 presidential campaign, he called for fresh thinking toward American adversaries like Iran. As president, he has argued that it is wiser to negotiate with a nemesis of more than three decades than risk a military confrontation over Iran’s nuclear ambitions.

But he has also insisted, in response to the many critics who called him desperate for a deal, that signing off on a weak agreement is not in his interest. “If Iran has a nuclear weapon, it’s my name on this,” the president said in May — a measure of just how keenly aware he is of the deal’s implications for his legacy.

The deal came 18 days after John Kerry arrived in Vienna for a round of talks whose initial deadline was June 30. As early July came and went, the talks became bogged down in a handful of issues, including whether and how to lift a U.N. embargo on Iran’s import and export of conventional arms.

Kerry, too, was accused of wanting a deal too badly, a charge his marathon session in Vienna — all of it on crutches, as he recovers from a broken leg — may have defused.

The comprehensive deal fills in the details of a preliminary framework deal reached in April by the U.S. and its negotiating partners.

The goal was to achieve a “breakout time” — or the time it would take Iran to produce the nuclear material required for one bomb — of at least one year. That is long enough, U.S. officials believe, for the world community to take punitive action that could include air strikes against Iranian facilities.

The comprehensive agreement will leave many powerful critics deeply dissatisfied, besides Israel, which feels that its security is being jeopardized; several Sunni Arab states, especially Saudi Arabia, worry that the end of sanctions will free up tens of billions of dollars that Iran can direct to militant allies like Hezbollah and sectarian battles from Iraq to Syria to Yemen.

The emerging outlines of the deal have also drawn withering fire for months from Capitol Hill, fueled by Republicans — and many Democrats — who agree with Netanyahu. A vote of disapproval by Congress would prevent Obama from lifting sanctions on Tehran, although administration officials believe they can maintain enough Democratic backing to sustain a presidential veto.

Punishing Tehran

The nuclear talks began after years of Western worry over Iran’s nuclear aims, despite Tehran’s insistence that its program was only for scientific and energy purposes.

The 2002 revelation of Iran’s uranium enrichment plant at Natanz and a plutonium reactor at Arak — sites previously undeclared to the U.N. in violation of Iran’s international commitments — badly undercut Tehran’s claims. So did the 2009 discovery of a secret, underground enrichment facility buried in a mountain at Fordow, near the holy city of Qom.

In 2007, an assessment by the U.S. intelligence community found that Iran had pursued a military dimension to its nuclear program — in effect, an Iranian Manhattan Project — but ceased the work in 2003. Iran has failed to comply fully with International Atomic Energy Agency inspectors pursuing the question, a major point of contention in the talks.

The United States, which broke off diplomatic relations with Iran after the country’s 1979 Islamic revolution and capture of the U.S. embassy in Tehran, has long used sanctions and other tools to punish Iran for its behavior.

In recent years the sanctions vice tightened considerably as Russia and China backed harsh U.N. measures, the U.S. Congress targeted Iran’s financial sector, and the Obama administration persuaded growing economies like India and South Korea agreed to sharply limit their consumption of Iranian oil.

At the same time, after taking off Obama continued a Bush-era program targeting Iran’s nuclear infrastructure with cyberwar, in the form of the Stuxnet virus, which delayed Iran’s enrichment program.

The U.S., U.N. and EU sanctions badly hurt Iran’s economy; inflation soared above 40 percent in 2013. U.S. sanctions threatening to punish foreign financial institutions for doing business with Iran landed a particularly painful blow.

In mid-2012, after exchanging messages through the Arab state of Oman, Iranian and American officials secretly met to lay the groundwork for potential nuclear talks. This involved diplomats dispatched by then-Secretary of State Hillary Clinton, including her close aide Jake Sullivan, now a top adviser to her presidential campaign.

Talks became possible in June 2013, after Iranians elected Rouhani, a relatively moderate cleric by Iranian standards, as president. He offered the west a more conciliatory face than his hardline predecessor, Mahmoud Ahmadinejad.

With the apparent blessing of Iran’s Supreme Leader Ayatollah Ali Khamenei, who has the final word on all state decisions in Iran, Rouhani spoke directly via phone with Obama in September 2013, kicking off the public phase of the nuclear talks.

Despite a willingness to bargain over its nuclear program, Iran has pursued an aggressive foreign policy elsewhere in the Middle East, alarming its neighbors.

For example, Iran is backing Syrian dictator Bashar Assad against Sunni groups trying to topple the dictator. At the same time, Iranian-trained Shiite militias are indirectly helping U.S.-backed Iraqi troops trying to defeat the Islamic State terrorist group in Iraq. And in Yemen, Iran has backed the Houthi rebels, whose positions are being bombed by Saudi Arabia with U.S. assistance.

Critics of the deal fear that once Iran starts to get relief from sanctions — including access to more than $100 billion in frozen assets — it will use the money to wreak more havoc in the region.

In many ways, the hardest part of the nuclear deal begins now with its implementation.

The U.S. Congress is likely to vote to prevent Obama from suspending sanctions on Iran — a measure Obama promised to veto. In that scenario, the votes of a few dozen Senate Democrats could become decisive in a veto override battle.

The nuclear deal will also drive the 2016 presidential campaign’s foreign policy debate. Already some Republican contenders have vowed to roll it back if they win the White House.

Michael Crowley contributed to this report.

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