development in the 1920s and 1930s After four years of warfare, the Red Army's defeat of Pyotr Nikolayevich Wrangel in the south in 1920 allowed the foundation of the Union of Soviet Socialist Republics in December 1922. Historian John Erickson sees 1 February 1924, when Mikhail Frunze became head of the Red Army staff, as marking the ascent of the general staff, which came to dominate Soviet military planning and operations. By 1 October 1924 the Red Army's strength had diminished to 530,000. The list of Soviet Union divisions 1917–1945 details the formations of the Red Army in that time. In the late 1920s and throughout the 1930s, Soviet military theoreticians – led by Marshal Mikhail Tukhachevsky – developed the deep-operations doctrine, a direct consequence of their experiences in the Polish-Soviet War and in the Russian Civil War. To achieve victory, deep operations envisage simultaneous corps- and army-size unit maneuvers of simultaneous parallel attacks throughout the depth of the enemy's ground forces, inducing catastrophic defensive failure. The deep-battle doctrine relies upon aviation and armor advances with the expectation that maneuver warfare offers quick, efficient, and decisive victory. Marshal Tukhachevsky said that aerial warfare must be "employed against targets beyond the range of infantry, artillery, and other arms. For maximum tactical effect aircraft should be employed en masse, concentrated in time and space, against targets of the highest tactical importance." Red Army deep operations found their first formal expression in the 1929 Field Regulations, and became codified in the 1936 Provisional Field Regulations (PU-36). The Great Purge of 1937–1939 and the Purge of 1940–1942 removed many leading officers from the Red Army, including Tukhachevsky himself and many of his followers, and the doctrine was abandoned. Thus at the Battle of Lake Khasan in 1938 and in the Battle of Khalkhin Gol in 1939 (major border clashes with the Imperial Japanese Army), the doctrine was not used. Only in the Second World War did deep operations come into play. Chinese–Soviet conflicts The Red Army was involved in armed conflicts in the Republic of China during the Sino-Soviet conflict (1929), the Soviet Invasion of Xinjiang (1934), when it was assisted by White Russian forces, and the Xinjiang rebellion (1937) in Northwest China. The Red Army achieved its objectives; it maintained effective control over the Manchurian Chinese Eastern Railway, and successfully installed a pro-Soviet regime in Xinjiang. Soviet–Japanese border conflicts The Soviet–Japanese border conflicts,