Why might the Bush administration pursue military action against Iran, even though virtually everyone agrees its a bad idea? Because of a combination of an arrogant Mideast policy and Iran's willingness to call out that policy.
Among the shifting reasons given for the Iraq war, the ones that have stood the test of time are these:
1. The grave terrorist threat to United States security can only be reduced in a decisive way by removing the undemocratic regimes in the Middle East which support those groups.
2. Undemocratic regimes in the Middle East are unpredictable and thus poor custodians of the strategically irreplacable oil supply. Safeguarding this supply is one of the real benefits of the world system involving American hegemony.
3. Populations in Middle East countries really want democracy in the Western style.
Based on these premises, there's a view within the administration that the proper course of action is to actively or clandestinely overthrow the existing governments in the Middle East and establish popularly-supported democracies which will be more congenial to the rest of the world.
But Iran has called that bluff. By insisting that the Bush administration provide it with security gaurantees, Iran is asking explicitly for the abandonment of the whole "domino" premise of the Iraq war: that a democracy in Iraq would be the seed for the overthrow of the governments of Iran, Syra, Jordan, Egypt, and so on.
But the Iraq war and its predicted geopolitical effects are the centerpiece of Bush foreign policy. How's the domino effect going to happen if the target states just negotiate an abandonment of that policy?
Will Bush agree? Or would he rather attack, counting on the opposition in Iran to support the U.S.? Iran must realize what it is asking for. Do we?