It's clear, under military law, that military members can be held accountable for crimes committed under the guise of "obeying orders," and there is no requirement to obey orders which are unlawful. However, here's the rub: A military member disobeys such orders at his/her own peril. Ultimately, it's not whether or not the military member thinks the order is illegal or unlawful, it's whether military superiors (and courts) think the order was illegal or unlawful.

http://usmilitary.about.com/cs/militarylaw1/a/obeyingorders.htm