The Adriatic Charter is an association formed by Albania, Croatia, Republic of Macedonia and United States of America for the purpose of aiding their attempts to join NATO. The Charter was signed on 2 May 2003 in Tirana under the aegis of the United States. The role of the United States has caused some confusion; in discussions in the other member states, the Charter is often called the U.S.-Adriatic Charter. In September 2008 Montenegro and Bosnia and Herzegovina were invited to join the Charter and joined on December 4, 2008. Serbia accepted observer status at the same time. On April 1, 2009, Albania and Croatia became the first of the group to join NATO.