Article I, Section 9, Clause 8:
No Title of Nobility shall be granted by the United States: And no Person holding any Office of Profit or Trust under them, shall, without the Consent of the Congress, accept of any present, Emolument, Office, or Title, of any kind whatever, from any King, Prince, or foreign State.
In 1871 the Attorney General of the United States ruled that:
A minister of the United States abroad is not prohibited by the Constitution from rendering a friendly service to a foreign power, even that of negotiating a treaty for it, provided he does not become an officer of that power . . . but the acceptance of a formal commission, as minister plenipotentiary, creates an official relation between the individual thus commissioned and the government which in this way accredits him as its representative, which is prohibited by this clause of the Constitution.1