The US has categorically denied any role in yesterday's assassination of an Iranian nuclear scientist or the recent deadly attack on an Iranian missile facility. But two American aircraft carrier battle groups are near Iranian shores with a third on the way, and the US is imposing new sanctions that could do real damage to Iran's oil economy. Is regime change of the current leaders of Iran's Islamic Republic America's real goal? Could that policy backfire, making Iran more determined than ever to build nuclear weapons? Will tough talk in the presidential campaign convince them there's no other choice?