During the Second World War Bulgaria never declared war against the Soviet Union.
However, in the period 1941-1944 the Soviet Union did conduct undeclared warfare against Bulgaria, especially along the Black Sea coast where Soviet submarines torpedoed Bulgarian ships and engaged in other less-than-friendly activities.
This, of course, belongs to history – or does it? Stalinist monstrosities glorifying the Red Army still dot the Bulgarian Black Sea coast – as they in fact do every chunk of Bulgaria, making it the only former Warsaw Pact country not to scrap them or put them in a museum.
Where in Bulgaria are you?