Cultural interactions between the Thracians and Hellenic civilization were particularly active during VI-II century BC. Many cities were founded in Thrace, Moesia and on the shores of the Black Sea that became conduits of Greek cultural influence. In the middle of the first century AD. all Bulgarian lands became part of the Roman Empire. Many architectural and archeological monuments have been preserved from this period - the Ancient Theater and the Roman Stadium in Plovdiv, the remains of the Roman cities of Ulpia Escus, Nove, Nikopolis ad Istrum, Nikopolis ad Nestum, Augusta Trayana, Abritus and others.
During the reign of Khan Krum (803-814) Bulgaria bordered on the west with the empire of Charlemagne, and on the east the Bulgarian troops reached the walls of the capital of Byzantium - Constantinople. In 864, during the reign of Prince Boris I Michael (852-889), the Bulgarians adopted Christianity as the official religion and Bulgaria became one of the oldest Christian countries in Europe.
At the end of the ninth century, the brothers Cyril (Constantine the Philosopher) and Methodius created and distributed the Slavic alphabet. Ohrid and Veliki Preslav became centers of Bulgarian and Slavic culture. From Bulgaria the Slavic script spread to other Slavic countries. Even today, countries such as Russia, Serbia, Ukraine, Macedonia and Belarus use the Cyrillic alphabet, whose spelling rules were created by the students of Cyril and Methodius and their followers in the Bulgarian capital Preslav. The reign of Tsar Simeon I (893 - 927) is known as the "Golden Age of Bulgarian culture", and the country's borders at that time reached the Black Sea, White Sea and the Adriatic Sea.
After the Second World War, Bulgaria fell into the sphere of political and economic influence of the USSR. In 1946 the country was proclaimed a republic. The Bulgarian Communist Party came to power. Political parties outside the Patriotic Front are banned, the economy and banks are nationalized, land is organized in cooperatives.