The key in the oath concerning this Marine is “I will obey the orders of the President of the United States”. Unless Congress says otherwise, I believe all his orders are lawful.
Thank you for the clarification and for providing the text of the oath for the discussion.
From the text of the oath, it doesn't sound like "I will obey the orders of the President of the United States" is the key part of the sentence. The statement to obey the president is qualified by the clause which follows it. In my opinion, the key part of the sentence is the part that says "I will obey the orders of the President of the United States according to regulations and the Uniform Code of Military Justice.
This clause implies that the officer only swears to uphold the orders of the president which are in accordance to the set regulations and rules of the Uniform Code of Military Justice. If a president's orders are not in conformance to the regulations there is no requirement to obey. An enlisted man does not swear to obey all orders, even if unlawful. In fact, to obey presidential orders which are not in accordance with the UCMJ would be a direct breach of the oath the enlisted man took.
Also, I think it is important to note that the Uniform Code of Military Justice is a law passed by Congress and all legislation passed by Congress must conform to the US Constitution. Therefore, if any rule or regulation in the UCMJ contradicts the US Constitution, the soldier is not bound to obey those either. At least, that is what the oath sounds like it is saying to me.
I some times wonder how many of our enlisted personnel actually understand or read the UCMJ or the Constitution. I will admit that when I was 18 years old and in the service, I didn’t know everything or understand everything in either one. I was too busy trying to learn what was required of me.