Bulgaria became a member of the United Nations in 1955 and since 1966 has been a non-permanent member of the Security Council three times, most recently from 2002 to 2003. It was also among the founding nations of the Organization for Security and Co-operation in Europe (OSCE) in 1975. Euro-Atlantic integration has been a priority since the fall of communism, although the communist leadership also had aspirations of leaving the Warsaw Pact and joining the European Communities by 1987. Bulgaria signed the European Union Treaty of Accession on 25 April 2005, and became a full member of the European Union on 1 January 2007. In addition, it has a tripartite economic and diplomatic collaboration with Romania and Greece, good ties with China and Vietnam and a historical relationship with Russia.