'Bulgaria and Russia are marching in one column under the banner of labour...' goes this song. The march was a favourite of the Soviet Army Group in Bulgaria.
In September 1944 the Red Army crossed the Danube and entered Bulgaria.
It ignored the announcement of the Bulgarian government that it was withdrawing unilaterally from the Axis, pulling its troops out of Greece and Yugoslavia and then declaring war on Germany, hoping to avoid a Soviet occupation. By the time the Red Army entered Sofia on 16th September 1944, the Bulgarian capital was already controlled by armed Communist activists. The Soviet army presence enabled the Bulgarian Communists to later seize power and establish a Communist state.
One of the first tasks for Soviet Power in Bulgaria was to establish continuity between the XIX Century Russian presence in Bulgaria and its new rulers. The myth of the Dual Liberators was born.
The new administration was equally harsh towards all within Bulgaria's borders. Tens of thousands of White Russians lived in Bulgaria after the 1917 Bolshevik Revolution. Legally, they were not Soviet citizens, some of them had Bulgarian passports, others the so called Nansen Passports (as stateless people in need of protection), issued by the League of Nations in Geneva.
Similarly to the post-1945 practice across Eastern Europe, some of those Russians were taken to the Soviet Union, others to Bulgaria's Forced Labour Camps: first the Kutsiyan Mine near the town of Pernik, later to Bulgaria's largest Gulag on the Danubian island of Persin.