The real talks on Iran have now begun. This is the significance of the first meeting, in Vienna this week, between Iran and six world powers led by the United States.

What went before, an interim framework agreement that caused a great deal of controversy, is no longer the bogeyman. That was a false bogeyman since it was merely a way of freezing Iran’s nuclear work in place so talks could be held, and yes, it left Iran with almost all of its nuclear capacity.

The genuine bogeyman now includes big questions with big consequences—how many centrifuges will Iran have to enrich uranium, ostensibly for civilian reactor use but also possibly to make bombs; if Iran will be able to use advanced centrifuges which can enrich more quickly; if Iran is free to develop ballistic missiles which could carry atom bombs; if Iran will have a plutonium-producing reactor; if Iran can continue to do nuclear work at the site of Fordow, buried under a mountain and so relatively invulnerable to air attack.

The talks on a so-called comprehensive agreement are set to last six months and will determine the future shape of Iran’s nuclear program, and whether the United States and Iran can agree on what is expected to be a more limited Iranian nuclear capability.

A senior U.S. administration official here said that “these negotiations are the best chance we’ve ever had for diplomacy to resolve this most pressing national security challenge.” The alternative to resolving it may very well be war, or at least an increased sanctions regime against Iran that would begin to look like economic war. The hardliners from the West, from Israel to the U.S. Congress, are standing down, giving the six months of diplomacy a chance. How things look by July will be crucial in determining if negotiations can continue, if necessary, or if the hardliners will use their influence and power to pull the plug on the peace process.

The meeting in Vienna from Tuesday to Thursday is being advertised as an agenda-setting exercise, even if the two sides were already feeling each other out on the major issues. And there was not much agreement according to sources close to the talks. As expected, and somehow little appreciated by critics of the interim accord, all the major issues are now in play. The U.S. official said in a talk with reporters that UN Security Council resolutions “have to be resolved before a final agreement.” On the agenda, since mentioned in the resolutions, will be “ballistic missiles capable of delivering nuclear weapons,” as well as calls on Iran to suspend uranium enrichment and allow for wider inspections, the official said.

Meanwhile, the Joint Plan of Action of the interim agreement also outlined wider issues to be discussed, such as the possible military dimensions of Iran’s nuclear work. The result: the limited nature of the limited agreement has given way to a full discussion on what sort of nuclear program Iran will have.

To some extent the two sides are on uncharted ground. The interim agreement was preceded by secret U.S.-Iranian talks to hammer the agreement out, and it was basically set by the time the Iran and the six negotiating states sat down in Geneva in October. This time, there have not been such U.S.-Iranian discussions. Said the U.S. official: “In the first step, as you all know, there was a behind-the-scenes, shall we say, bilateral track that went on for some time and then was folded into the P5+1 [the six powers the United States, China, Russia, Britain, France and Germany] negotiations.  This time we’re not repeating that process.”

Iranian leaders have been vocal in saying that they will not dismantle parts of their program, as would be needed to guarantee that Iran could not “break-out” quickly to make the fissile material needed for a bomb. There are in fact hot-button issues, red lines of a sort, which will determine whether an eventual deal is palatable. For the United States these would almost undoubtedly include Iran sharply reducing the number of centrifuges it has both actually enriching, some 10,000, and installed, a total of some 19,000; reducing its stockpiles of enriched uranium; stopping production line enrichment at Fordow; not using advanced centrifuges; not finishing work on the Arak heavy-water reactor; and guaranteeing that it would not develop ballistic missiles that could deliver a nuclear strike. Iran will be resisting all these measures, as it did in Vienna, according to sources close to the talks.

Does this doom the talks, as Iran’s Supreme Leader seems to think is the case? Not necessarily. The fact is that an eventual deal could consist of unsatisfactory compromises on key issues but result in a package which could be sold to hardliners in and allied with both Washington and Tehran. The Obama administration, for example, might be going to Congress in July, or next January if talks run a second six months, to say: “Look, you are clearly unhappy with many of the details but the bottom line is that this package will keep Iran from breaking out in less than a year to make a bomb and will present a solution short of war.” If the deal clearly does this, it will be hard for Congress to reject and move towards confrontation with Iran.

This is the optimistic scenario. The more pessimistic alternative is that there will be no such acceptable package, talks will break down and we will return to a dynamic of confrontation and possible escalation. This will depend on the power calculations on both sides. Will sanctions already in effect continue to torpedo the Iranian economy, or will sanctions begin to crumble? And there is the question about whether the United States will hang tough in demanding a reduction in Iran’s nuclear means, or yield to too many compromises.

This article originally appeared on The Daily Beast.