Complex sentence: During the final decades of the Russian Empire, the port city of Baku, became a large metropolis due to the discovery of Oil in the Caspian Sea, by the 1930s, it was the capital of the Azerbaijani SSR and the largest city in Soviet Transcaucasia.
Split into simpler sentences: During the final decades of the Russian Empire the port city of Baku became a large metropolis due to the discovery of Oil in the Caspian Sea. By the 1930s, it was the capital of the Azerbaijani SSR and the largest city in Soviet Transcaucasia.