Fully independent Bulgarian kingdom (Tsardom) from Ferdinand I's 1908 declaration of independence, through the Balkan Wars, WWI, and WWII as an Axis-aligned state under Boris III and then the child-tsar Simeon II's regency.
Monarchy abolished by plebiscite in September 1946; the People's Republic of Bulgaria was formally proclaimed shortly after, forcing Simeon II into exile.
The atlas holds 5 border frames for Tsardom of Bulgaria — the shape you scrub through changes in 1908 · 1934 · 1940 · 1941 · 1945.
Cambridge's standard concise history of Bulgaria, covering the Tsardom's WWII Axis alignment and collapse for a general reader.