From the Document: "Bulgaria is a Southeastern European country located along the Black Sea. After decades of single-party communist rule and a turbulent transition period during the 1990s, Bulgaria joined NATO in 2004 and the European Union (EU) in 2007. The United States cooperates with Bulgaria on a range of security issues but also has raised concerns over corruption and the rule of law."
CRS In Focus, IF11725
Congressional Research Service: https://crsreports.congress.gov/