Can they be punished for this? They certainly can. An order to perform a dangerous mission is lawful because it's not an order to commit a crime. Under current law, and the Manual for Courts-Martial, "An order requiring the performance of a military duty or act may be inferred to be lawful and it is disobeyed at the peril of the subordinate. This inference does not apply to a patently illegal order, such as one that directs the commission of a crime."
Probably the most famous case of the "I was only following orders" defense was the court-martial (and conviction for premeditated murder) of First Lieutenant William Calley for his part in the My Lai Massacre on March 16, 1968. The military court rejected Calley's argument of obeying the order of his superiors. On March 29, 1971, Calley was sentenced to life in prison. However, the public outcry in the United States following this very publicized and controversial trial was such that President Nixon granted him clemency. Calley wound up spending 3 1/2 years under house arrest at , where a federal judge ultimately ordered his release.
Military discipline and effectiveness are built on the foundation of obedience to orders. Recruits are taught to obey, immediately and without question, orders from their superiors, right from day one of boot camp.
Military members who fail to obey the lawful orders of their superiors risk serious consequences. of the makes it a crime for a military member to WILLFULLY disobey a superior commissioned officer.
I do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; and that I will obey the orders of the President of the United States and the orders of the officers appointed over me, according to regulations and the Uniform Code of Military Justice.
These articles require the obedience of LAWFUL orders. An order which is unlawful not only does not need to be obeyed but obeying such an order can result in criminal prosecution of the one who obeys it. Military courts have long held that military members are accountable for their actions even while following orders -- if the order was illegal.
The Vietnam War presented the United States military courts with more cases of the "I was only following orders" defense than any previous conflict. The decisions during these cases reaffirmed that following manifestly illegal orders is not a viable defense from criminal prosecution. In United States v. Keenan, the accused (Keenan) was found guilty of murder after he obeyed in order to shoot and kill an elderly Vietnamese citizen. The Court of Military Appeals held that "the justification for acts done pursuant to orders does not exist if the order was of such a nature that a man of ordinary sense and understanding would know it to be illegal.
So, to obey, or not to obey? It depends on the order. Military members disobey orders at their own risk. They also obey orders at their own risk. An order to commit a crime is unlawful. An order to perform a military duty, no matter how dangerous is lawful, as long as it doesn't involve the commission of a crime.