When you are already trying to hold your own against a couple of bad-asses in the meanest part of town, the last thing you need is for your drunken girlfriend to go start something with the biker gang on the corner.
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.
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." [I and my Nellis komrads concur. -M1]
But that doesn't take into consideration that the Defense Department is the largest and most complicated department in the US government. As a bureaucracy, the Pentagon is almost Ottoman in terms of its scale and complexity. The system is dependent on thousands of mid-level military officers and civilian bureaucrats, and if a few determined bureaucrats set their minds to slowing a march to war, they can do it. The employees of the Pentagon can insist that every form be filled out in triplicate, that every authorization be approved by Congress, and bury those agitating for war in so much paperwork it would take a determined effort just to dig out.
The second thing those military officers and bureaucrats could do is leak information to the press should the administration begin taking secret steps toward military action. Prior to the Iraq war, officers rarely came into contact with journalists. But thanks to the personal relationships that have developed between journalists and the military in Iraq and Afghanistan, nearly every mid-level US Army or Marine Corps officer has the name and number of at least one journalist in his or her rolodex.