The joint Chiefs are all political appointments and if the president doesn't like their expert opinions then if he says he wants their official opinion should be that a military strike will do no good then that will become official policy of the Joint Chiefs. All they can do is behind the scenes try talk Obama into taking a realistic line based on their advice and not just one designed to rubber stamp his political policies. Military men who are in their jobs because of political apointments are not military men we should consider independent experts.
IDF reservist, 44, dies of heart attack during training (Haaretz)
from the article: 'Military strike won't stop Iran's nuclear program'