"By mid-1921, the Red Army had overcome opposition in the Caucasus and Central Asia, although its invasion of Poland in 1920 was driven back. These conflicts were linked to rivalry between the great powers. The conflict in the Caucasus was in part an instance of the struggle between Britain and the Soviet Union. The British saw Armenia, Azerbaijan and Georgia, each of which briefly became independent in the Russian Civil War, as a buffer for their interests in Iraq, Persia (Iran) and India; and as a source of raw materials, notably oil from Azerbaijan and access to oil from Georgia. In late 1918, the British landed troops in the Black Sea port of Batumi, the terminus of the railway to the oil-producing centre of Baku on the Caspian Sea. This was a commitment advocated by Mackinder. Torpedo-armed coastal motor boats were sent overland to the Caspian Sea. However, under pressure from too many commitments, the British withdrew their forces from late 1919. Benefiting from the divisions between the Caucasus republics, the Soviets advanced and took them over in 1920–1."