 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 . Bulgaria was also among the founding nations of the Organization for Security and Co @-@ operation in Europe ( OSCE ) in 1975 . It joined NATO on 29 March 2004 , signed the European Union Treaty of Accession on 25 April 2005 , and became a full member of the European Union on 1 January 2007 . Polls carried out seven years after the country 's accession to the EU found only 15 % of Bulgarians felt they had personally benefited from membership , with almost 40 % of the population saying they would not bother to vote in the 2014 EU elections . Euro @-@ Atlantic integration became a priority for the country since the fall of Communism , although the Communist leadership also had aspirations of leaving the Warsaw Pact and joining the European Communities by 1987 . 
