Andrew Exum offers reasons against war on Iran, from a military perspective:
Leaving aside the relative merits of a strike against the Iranians, why might America's military resist such action? First, consider the fact that the US has at the moment 162,000 troops in Iraq, 30,000 in Kuwait, 4,500 in Bahrain and 3,300 in Qatar - not to mention the two carrier battle groups in the Gulf or the 8,500 troops on the ground in Afghanistan. In the event of an American or Israeli strike against Iran's nuclear facilities, for example, the troops in Iraq, the Gulf and Afghanistan would be in even greater danger than they already are, vulnerable to an Iranian counterattack or, more likely, an Iranian-sponsored terror campaign.Can't we just have a military coup right now, please?
Second, there exists a tremendous sense of guilt among the US senior officer corps for what is seen as a failure to stand up to the civilian leadership in the rush to go to war against Iraq in 2002 and 2003. Much of the current divide between America's generals and its junior officer corps boils down to a sense on the part of junior officers that their superiors largely acquiesced to whatever Defense Secretary Donald Rumsfeld said in the run-up to the Iraq war. The charge of spinelessness is one that hurts America's generals, especially as it comes from lieutenants and captains who have proven themselves on the battlefield of Iraq.
Third, in the wake of the Iraq war, professional military officers are more suspicious than ever of think-tank types with theories on how easy military victories can be achieved. As an active-duty US Army officer recently told me: "If I hear one more lawyer with no military experience explain to me how air power alone really can do it this time, I'm going to kill him."