Several times Iran postponed the resumption of uranium conversion, which can be a step towards fashioning a nuclear bomb, while Britain, France and Germany, with American support, worked on a package of economic and political incentives for Iran to give up its nuclear plan.

NPR: Iran's Leader May Favor a Showdown

The HEU Agreements provide for the conversion of approximately 500 metric tons of highly enriched uranium contained in Russian nuclear weapons into low-enriched uranium for use as fuel in commercial nuclear reactors.

WHITEHOUSE: Executive Order--Russian Highly Enriched Uranium