The Logan Act is a 1799 federal law (amended in 1994) prohibiting private citizens from conducting diplomatic relations with a foreign government.
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.
In April 2007 a controversy erupted over whether Speaker of the House Nancy Pelosi violated this law by visiting the head of state of Syria, Bashar Assad, who has been diplomatically shunned by the Bush Administration.