The regulations (which stem from the law of the land) require you to carry out the mission as briefed. There is a caveat which applies in peacetime only, which says something about due regard for the safety of the crew, passengers and persons below. In war you get on with it. If you can't take a joke, you shouldn't have joined. This does leave room for the lawyers to argue what constitutes a war. I would hope that the courts would take a broad view - if we ain't prepared to go, what have the taxpayers been wasting their money on?