Under the Logan Act..."Any citizen of the United States, wherever he may be, who, without authority of the United States, directly or indirectly commences or carries on any correspondence or intercourse with any foreign government or any officer or agent thereof, with intent to influence the measures or conduct of any foreign government or of any officer or agent thereof, in relation to any disputes or controversies with the United States, or to defeat the measures of the United States, shall be fined under this title or imprisoned not more than three years, or both."
Senator Thomas Cotton and 46 of his fellow Republican Senators have violated this act with their letter to the leaders of Iran, telling them any treaty reached between them and the US President would not be honored.