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, and says an attack on Iran is not inevitable. But two American aircraft carrier battle groups are near Iranian shores with a third on the way, and the Pentagon is making contingency plans in case Iran learns to make nuclear weapons. Diplomacy is the official policy, but the US is imposing new sanctions that could do real damage to Iran's oil economy, and State Department officials say they're "tightening the noose." If Iran's rulers think America's goal is regime change, will they be more likely to develop atomic bombs for their own protection? Is anti-Iranian rhetoric by Republican candidates pushing the President to take a harder line that will make confrontation more likely than diplomacy?