Russia, under its autocratic leader Josef Stalin, believed that it had a right to the Eastern European nations it had occupied in World War II. After being invaded by Germany in two consecutive wars, the USSR thought it imperative that buffer states be created to protect the borders of the fatherland. With Communist regimes in place, the nations of Eastern Europe could be controlled by Russia and, by their location, protect it. In that sense, at the same time that we talk about decolonization after the WW II, we should also point out that a new colonization began - the Russian colonization of Eastern Europe, namely of Poland, Hungary, Romania, Czechoslovakia, and Bulgaria. In the case of Bulgaria, after the Yalta Conference, USSR had 90% control and influence over the country, which meant virtually total control in all aspects.
However, this control was not forceful. During the Cold War, Bulgaria sought and earned the title of the "Soviet Union's Most Loyal Ally." Georgi Dimitrov, one of the most important Bulgarian communist leaders, coined the slogan that Soviet friendship is Bulgaria's "air and sun," and Todor Zhivkov, the Bulgarian Communist Party leader, often stated that "the Bulgarian watch is set on Moscow time." Signs of Bulgaria's fidelity to the USSR abounded. The Soviet ambassador regularly took part in important state occasions and rituals alongside members of the Politburo and other high Bulgarian officials; when Dimitrov died, he was laid to rest in a mausoleum modeled on Lenin's; and in the 1960s, a verse celebrating solidarity with Moscow was inserted in the national anthem.
Bulgaria's domestic policies were as closely modeled on the Soviet example, as was the postwar architecture of downtown Sofia, the capital. Within the communist world and on the larger international stage Bulgaria served as a Soviet echo, defending the USSR against Western criticism, to the extent that Western observers took to calling it the "sixteenth Soviet republic." In fact, in 1963 Zhivkov proposed to Khrushchev that Bulgaria actually be incorporated into the USSR. Even though the Soviet leader rejected the offer, Bulgaria's development was closely integrated with the Soviet economic system.
This relationship that developed between Bulgaria and the USSR after WW II owed to hard historical realities. The Bulgarian communist regime, in its full Stalinist variant, was installed with great brutality by men and women who had spent much of their liver in the USSR and who were closely monitored by Soviet authority. The new Bulgarian constitution was drafted by Soviet experts and reportedly edited by Stalin himself. In addition, the party leader, Todor Zhivkov, owed his elevation to Khrushchev's direct backing.
Interesting little facts: When I was in elementary school, before learning the names of Bulgaria’s national heroes, we first learned about Yuri Gagarin, the Russian man who first flew in outer space. Of course, Russian was obligatory to study in schools. Moreover, books in Bulgaria were translated from Russian: for example, an Albert Camus book, The Outsider, was translated not from its French original, but from its Russian translation. This was the case with all books, and this is the reason why in Bulgarian we pronounce the names of countries, cities, etc, the way Russians do. We had two TV channels: one Bulgarian (the Bulgarian National TV), and one Russian. Foreign movies for Bulgaria meant Russian movies.