Sometimes called the First Rule of Command, "Never give an order you know will not be obeyed," is one of the fundamental principles of officership. Command is a relationship, and must be cultivated as such - and, in many fashions, it is built on the trust a subordinate has in their commander.

Once a commander has given an order they know will not be obeyed, they open the door to questioning orders when it really matters. When giving an order that is almost certain to result in the deaths of those ordered, the absolutely last thing you want them thinking is, "Hey, remember how we already disobeyed him once? Maybe this would be a good time to do it again." Bad plan.

In general, orders that will not be obeyed involve personal issues, and may tend to be chickenshit orders. There isn't a hard and fast rule that you can apply, but use common sense. If your airman's wife just ran off to Las Vegas, NV with a jarhead, the order, "Let her go" is pretty likely to be ignored. Cover for him.