In September 2009, the U.S. intelligence community confirmed that Iran was continuing to build a covert uranium enrichment facility.
Undeterred, negotiators attempted again to engage Iran. In October 2009, western countries cobbled together a proposal known as a fuel swap.
Iran claims it needs to enrich uranium for "medical purposes." Under the deal, Iran would give up much of its less-enriched uranium and other countries would swap it for uranium enriched to the necessary level of purity.
Iran rejected this offer.
In February 2010, Iran announced that it would enrich the uranium itself. Obviously it was interested in having the ability to enrich uranium, not just having the finished product for medical purposes.