"Since May more details have been released, apparently, as many as Mossad feels can be safely revealed without exposing intel techniques and sources in Iran that the Iranians dont know about. Early on it was revealed that Mossad discovered the location of the warehouse containing the Iranian nuclear archive in early 2016 and by 2017 had put into motion a plan to get into the warehouse and copy key documents and get the information back to Israel. As soon as the warehouse was found it was put under an increasing degree of surveillance in an effort to find out exactly what was in there (32 massive safes and it took a while to find out which one contained the nuclear documents). While this was going on conditions were changing in Iran. There were more public demonstrations against corruption and government incompetence. The extent of these new attitudes became clear after news of the Mossad operation were made public. Most Iranians didnt care about the document theft. No Iranians were injured in the operation to the average Iranian it was another example of their governments incompetence.
Once inside the Mossad agents found far more documents than expected in the nuclear safe and were ordered to take 55,000 pages of paper documents and 183 CDs of containing over 50,000 files full of more data (including photos, engineering plans and so on) and get it back to Israel. The extent of the nuclear archive and the fact that many items appeared recent indicated that there was more in that safe than anyone expected. The extent of the information Mossad was able to obtain about the warehouse and the fact that the two Mossad vehicles carrying the documents were able to make it 500 kilometers to the Azerbaijan border without getting caught made it clear that attitudes had changed in Iran. The police discovered the warehouse break-in shortly after the Mossad agents and their two vehicles left. Iran then alerted over 12,000 security personnel to quietly chase down the perpetrators. The Iranians did not know the Israelis were headed for Azerbaijan, where a plane was waiting to get the documents back to Israel immediately.
The stolen documents detail how Iran had an active nuclear weapons program called AMAD from 1998 until 2003 but suspended it in early 2003 when the Saddam government was overthrown. The Iranian nuclear program was expensive and existed mainly because Saddam had convinced Iran that the Iraqi nuclear program was continuing in secret (because of UN sanctions). Once Saddam was out of power, and especially after he was captured and eventually admitted that the Iraqi nuclear program didnt really exist. Saddam kept enough of the defunct nuclear program around to scare the Iranians. Once Saddam was out of power the Iranian government found their program hard to justify.
That cancellation of the AMAD program was never completed because by the end of 2003 Iran changed its mind and converted AMAD into the stealthier SFAND program. This was a major deception because part of this AMAD/SFAND conversion involved going public with most aspects of the AMAD program that could be passed off as non-weapons related nuclear research. The weapons aspects went deep undercover and often literally deep underground. Iran (unofficially) and many Western politicians and intel officials (officially) insist that the archive only shows that the Iranians were trying to preserve the work they had done during the AMAD program and SDAND was merely an effort to keep AMAD updated. Israel disagrees and as more documents are translated and scrutinized the Israelis appear to be right."
https://www.strategypage.com/htmw/htchem/articles/20181018.aspx