(CNN) It is the moment that all the grunting and yelling has been leading up to.
Few doubted the United States and Iran deep-down preferred diplomacy -- even a new deal -- to escalating tensions, and possibly military confrontation. Now the moment of truth is here, in a Vienna conference room, where the E3+2 of the nuclear deal (UK, France, Germany, Russia and China) and Iran will sit down and talk about what comes next.
The Biden administration will be in the wings, spiritually if not always physically. US officials will be in Vienna, though they will not meet directly with their Iranian counterparts. Virtually every member of President Joe Biden's national security core is steeped in making the 2015 Iran agreement happen. Even the CIA director Bill Burns may well have got his job because of how well he knew the Iran negotiating brief. Of all the foreign policy challenges the US faces now, after the Trump administration's hiatus from normalcy, it is the crisis with the easiest solution at hand.
What of the Tehran moderates on the other side of the table? This too is their last real chance to make their broad proposal of better engagement with the western world work. It is true that Iran has begun to weather sanctions, or at least find a way to live badly with them. It's alleged that China has proven happy to buy the oil the rest of the world will not, taking up to a million barrels a day in March, according to some estimates measuring the "ghost tankers" that hide the shipments. China recently signed a significant trade deal with Iran, and has long been critical of US policy towards it. But few can pretend the damage being done by the US-led economic squeeze wouldn't be better off removed.
The moderates are also on the clock, facing a harder-line parliament, and a presidential election in months that could impose a tougher approach against the West across government. The material promise of the JCPOA -- as the nuclear deal is technically known -- needs to loom into view again, and quickly, to raise electoral turnout in their favor.
Both sides too have been doing what they can to prepare -- or place obstacles on -- the ground for Vienna. The Biden administration has been calm, and relatively unfussed by Iran's screaming deadlines and declarations of further nuclear activity. Biden took singular and swift military action when US military lives were lost and threatened in Iraq, by bombing Iran-backed militia. But it then swiftly started calling them "Shia-backed militia", as if to give everyone an option to step away from the smoking craters. The US has also declared the desire to bring some of the Trump-surge of troops protecting Saudi Arabia home. That must calm some hawks in Tehran.
The Iranians have deftly trod a line between enrichment actions that can terrify if you choose to let them, or be interpreted as symbolic and reversible if you prefer to remain calm. Enriching uranium to 20% purity -- as they have done -- rings alarm bells, because it is a closer step to the 90% needed for a bomb. But some estimates suggest that even if their 20% pile was enriched to upwards of 90% overnight, it would have enough for less than a handful of bombs. That's probably not enough of a reward to make a likely Israeli and US attack worth gambling over.
Iran's production of uranium metal is a necessary step towards bomb-making, critics have said. But they have been making that metal from low-grade uranium, so the step is mostly a show of a technical ability many experts already thought they had. Reports of more and better centrifuges also abound. In short, the Iranians have created a lot of chips to bring to the table -- a lot of things they can offer to do first, in exchange for the first softening of US sanctions.
These are, on both sides, serious people, and this is how negotiations go. The central calculation from both parties is as it was six years ago: that Iran would prefer sanctions relief over having the bomb. And that the US would prefer a richer Iran to a nuclear Iran. And that both sides would prefer not to have a war.
You can always get very excited about conflagration in the Middle East, as if it is always just a shot away. Yet wars are complicated, expensive, unpredictable, and tend to require both sides involved wanting or needing them to happen. Both Iran and the US do not want a war right now, palpably, or else they would have used one of the many opportunities over the past months or years to start one. So talks are the likely way forward.
Whatever comes from Vienna's meeting, it was conjured quickly after months of loud noises, and so is likely to have the foundations for it in place. A western source close to the talks told me there was, at a guess, an "8 out of 10" chance they would lead to further negotiations.
Vienna's meeting may not lead to a renewed JCPOA this week. It may never lead to the wider "JCPOA Plus" deal Biden wants, to rein in Iran's missile program and regional influence. It may not rescue Iran's economy before the presidential elections.
But it is the only real way forward, as both sides knew all along, and it is finally underway.