Complex sentence: Makhno's occupation of the southern Ukraine, especially the regions bordering on the Sea of Azov, soon posed a threat to General Denikin's entire offensive, as the supply base of Denikin's army was located in the region between Mariupol' and Volnovakha.
Split into simpler sentences: Makhno's occupation of the southern Ukraine, especially the regions bordering on the Sea of Azov, soon posed a threat to General Denikin's entire campaign. In fact, the supply base of Denikin's army was located in the region between Mariupol' and Volnovakha.