In the face of economic sanctions and international condemnation, Iran remains defiant over its nuclear energy program. It has threatened to close the Strait of Hormuz, a major passageway for much of the world's oil, in retaliation for any embargo. And it has vowed to punish the United States and Israel, accusing them of being connected to the recent assassinations of some Iranian nuclear scientists. Tehran insists it's pursuing nuclear energy for civilian purposes, not for military use. The U.S. and its allies aren't buying it.