Iran and world powers embark Tuesday on the Herculean task of transforming their interim nuclear deal into a long-term accord satisfying all sides and silencing talk of war for good. After a decade of failure and rising tensions, US President Barack Obama has put the chances of an agreement at "50-50", while Iranian Foreign Minister Mohammad Javad Zarif has predicted "difficult" discussions.
Getting a deal could also help bear fruit in other areas, not least in the three-year-old Syrian civil war where Tehran has staunchly backed President Bashar al-Assad. The latest peace talks ended in acrimony in Geneva on Saturday.
The scheduled three-day meeting in Vienna between Iran and the United States, China, Russia, Britain, France and Germany - the so-called P5+1 - is the first in what is expected to be a series of tricky encounters in the coming months. It comes after foreign ministers struck a breakthrough deal in Geneva on November 24 that saw Iran agree to curb - for six months - some of its nuclear activities in exchange for minor relief from painful sanctions.
That agreement, which came into force on January 20, extends the theoretical "break-out time" needed by Iran - which denies seeking the bomb - to produce enough highly-enriched uranium for a nuclear weapon. In return Iran is due to get over the six months some $6-7 billion in sanctions relief, including $4.2 billion in assets frozen in overseas bank accounts. It was also promised no new sanctions. But Iran's freeze is only temporary - although it can be extended - and the bulk of sanctions remain, continuing to deprive Iran of billions of dollars in oil revenues every week.