Iran quietly cooperated with the U.S. against the Taliban after 9/11, but President Bush's January 2002 "Axis of Evil" speech chilled any rapprochement. Later that year an exile group exposed secret enrichment and heavy-water sites, opening the nuclear crisis that dominates the rest of the story.
Whether Iran sought a bomb remains disputed. U.S. intelligence and the IAEA have long distinguished between enrichment capability and a decision to build a weapon, and intent versus leverage or latency is central to every negotiation.