A bunch of foreign policy luminaries has signed a report noting the high costs and limited benefits of military action to prevent an Iranian nuclear weapon. No former Secretary of State or Defense is on the list, but it includes Rich Armitage, Nick Burns, Tom Pickering, Tony Zinni and Sam Nunn.
The group says military action should be a last resort. It notes that the objective of preventing Iran from developing nuclear weapons is unlikely to be achieved by military action limited to air strikes, cyber attacks, and covert operations. When toting up the various costs, it expects retaliation against the United States and Israel, globalpolitical and economic instability, a breakdown of the international solidarity against an Iranian weapon, and "increased likelihood of Iran becoming a nuclear state."
This analysis doesn't make military action unthinkable, just risky and costly.