Complex sentence: After defeat of the Denikin's forces in 1920, it was returned to the borders that it had within the Russian Empire, while some eastern counties were transferred to the newly created Donetsk Governorate.
Split into simpler sentences: After defeat of the Denikin's forces, it was returned to the borders that it had within the Russian Empire. In 1920 some eastern counties were transferred to the newly created Donetsk Governorate.