The fear is that Iran, once the United States attacks it, will be faced with the Hobson's choice – either fire its missiles, it's main non-obsolete weapon, or risk the U.S. destroying them. Firing them risks conflagration of much of the Persian Gulf oil shipping infrastructure. New technology in cheap mines and drones might also close the Gulf to shipping. This would devastate the world economy much of which depends upon the oil.
Add to this America's penchant for starting wars, very often based upon lies or false premises. False flag operations mean having an attack blamed upon another nation which actually was not the culprit. Syria's questionable fault for gassing of innocent civilians was used by Washington to bomb its Air Force. Now Secretary of State Pompeo has set us up by publicly stating that any attack on any American soldier in Iraq or Syria would cause America to attack Iran. He's put every soldier at risk of death by some Israeli or Saudi fanatic.