The Russian diplomats knew that Bulgaria would not remain within these borders for very long — the San Stefano peace was called "preliminary" by the Russians themselves. The Berlin Congress began on and ended on with the Berlin Treaty that created a vassal Bulgarian state in the lands between the Balkans and the Danube. The area between the Balkan Mountains and the Rila and Rhodope Mountains became an autonomous Ottoman province called Eastern Rumelia. The separation of southern Bulgaria into a different administrative region was a guarantee against the fears expressed by Great Britain and Austria-Hungary that Bulgaria would gain access to the Aegean Sea, which logically meant that Russia was getting closer to the Mediterranean.