WASHINGTON (AP) -- The talks themselves were a groundbreaking and risky proposition when U.S. and Iranian officials met secretly in the sleepy Arab kingdom of Oman, archenemies feinting for a diplomatic opening. The opposing sides had barely spoken to one another in three-plus decades.

But after a torturous 2 1/2-year effort full of false starts, backward steps and missed deadlines, world powers and Iran transformed those early overtures into a nuclear accord that may reshape the security landscape of the Mideast for a generation to come.

Tuesday's agreement in Vienna, hashed out among seven nations in all, appeared in jeopardy several times even as the personalities changed and disputes evolved. The Americans and the Iranians, and the French and Russians, all added hiccups to the process. In the most recent round of discussions, negotiators busted past three target dates.

The final push, which encompassed 18 days of talks and seesawed between optimism and pessimism, served as a microcosm of the diplomacy.

The round started with great energy as diplomats raced to wrap up their work within four days. Reality quickly set in; negotiations slogged on for days with no end in sight. Chinese, European and Russian foreign ministers came and went, leaving most of the work to the American delegation under Secretary of State John Kerry and the Iran team under Foreign Minister Mohammad Javad Zarif.

"Sleepy and overworked," Zarif said the day before the accord.

"Believe me," Kerry said when the talks were over, "had we been willing to settle for a lesser deal, we would have finished this negotiation a long time ago."

In early 2013, few would have even imagined such a scenario.

President Barack Obama, fresh into his second term, asked his advisers to think big about a foreign policy agenda largely overshadowed in his first four years by the economy, health care and other domestic issues.

Iran was at the top of the list, but the signs pointed toward continued hostility and possibly war.

The Iranian nuclear program was inching closer toward nuclear weapons capability. The U.S.-led campaign of international penalties was crippling Iran's economy. Iran and the West were supporting opposite sides in Syria's civil war. Israel was threatening military action of its own against the Islamic Republic. The talk from Tehran and Washington was of animosity, not cooperation.

The diplomatic path out of the crisis started with a gamble from both sides - direct talks.

Brought together to resolve the fate of three American hikers held captive by Iran, U.S. and Iranian diplomats met on neutral ground in the Omani capital of Muscat in 2012 and again in 2013 to test whether the opportunity for a broader exchange was possible. Little was accomplished in those early meetings. But it became clear that the threat of a nuclear-armed Iran or a U.S. military attack needed to be confronted first.

Perhaps more important, the U.S. realized it might be able to deal with a government it considered the leading state sponsor of global terrorism. Iran discovered it could speak with a country its officials still sometimes called "the Great Satan."

Iran's citizens altered the equation by electing moderate-leaning Hassan Rouhani as president, in the summer of 2013, on a promise of compromise.

U.S.-Iranian talks quickly gained speed. But when America's Mideast allies and members of Congress realized what was going on, many were outraged, as were Iran's hard-liners.

By the time of Rouhani's historic telephone conversation with Obama in September 2013, American and Iranian diplomats had already sketched out in private the parameters of a grand compromise that would lock in place Iran's enrichment of bomb-making material and other nuclear activity in exchange for billions of dollars' worth of relief from economic sanctions.

Negotiators struggled for two months to unite on an approach for putting in place a "joint plan of action" reached in November 2013. In a scramble to complete the entire agreement within six months, the challenges began to come in from all sides.

At one point, Zarif walked out of the talks. He wasn't gone for long, however.

Congress threatened to disrupt the diplomacy with new Iran sanctions; Obama kept lawmakers at bay.

Israel led a fierce international lobbying campaign against any long-term accommodation of Iran's nuclear activity, but failed to sway the Obama administration.

Deadlines for a comprehensive pact slipped - in July 2014 and last November, and again three times in the past two weeks.

After an April framework, the U.S. and its partners faced backtracking from Iranian negotiators and the wrath of several defiant speeches from Ayatollah Ali Khamenei, the Islamic Republic's supreme leader.

That deal itself had come 48 hours after a final deadline, and disputes among the world powers were part of the problem. France clamored for tougher commitments from Tehran. Russia balked at sanctions provisions that would see it potentially lose its U.N. veto power.

Obama, himself under pressure, yielded shortly afterward to the overwhelming demand of American lawmakers for a post-deal, congressional-review period during which he would be prevented from making good on any concessions to the Iranians.

After a last, high-level preparatory meeting in May before the final push, Kerry broke his leg while bicycling in the French Alps.

Faced with serious divides - over inspections, Iranian research and development of advanced nuclear technology, the future of the U.N. arms embargo on Iran, and other matters - Kerry briefed the media on July 4 weekend and revealed his doubts.

The talks "could go either way," Kerry warned ominously, before hobbling on crutches back to the 19th century Austrian palace hosting the talks.

Five days later, he returned and threatened to abandon the talks unless Iran made a series of tough choices, prompting Iran's Zarif to accuse the U.S. of backtracking.

The mood soured. On several occasions, tempers flared and voices were raised.

By the beginning of this week, the gaps had been whittled down to a remaining few. These were finally bridged in a meeting that started with Kerry, European Union foreign policy chief Federica Mogherini and Russian Foreign Minister Sergey Lavrov.

Zarif then joined the discussion.

A half-hour thereafter, the ministers emerged and told aides that after 18 days of often fractious negotiation, they were satisfied.

By The Associated Press