§3331. Oath of officeThe duty of an officer in the uniformed services "to support and defend" is not to any particular person nor to a group of people nor to a state nor to any service. The Oath is to the foundational document of the United States - its Constitution.
An individual, except the President, elected or appointed to an office of honor or profit in the civil service or uniformed services, shall take the following oath: "I, AB, 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; that I take this obligation freely, without any mental reservation or purpose of evasion; and that I will well and faithfully discharge the duties of the office on which I am about to enter. So help me God." . . .
Interesting look at the history of this Oath here: