"Legal orders" are defined by the country giving them (in this case the US) which are a combination of the various treaties that the US has signed and laws. For

"Legal orders" are defined by the country giving them (in this case the US) which are a combination of the various treaties that the US has signed and laws. For instance, no soldier can legally be ordered to violate the Geneva convention, and they can be court martial for following such an order. A war of aggression is about international law, and is commonly not considered something that determines if an order is legal in any military.