The Sino-Soviet conflict of 1929 (, ) was an armed conflict between the Soviet Union and the Chinese warlord Zhang Xueliang of the Republic of China over the Chinese Eastern Railway (also known as the CER). The conflict was the first major combat test of the reformed Soviet Red Army, which was organized along the latest professional lines, and ended with the mobilization and deployment of 156,000 troops to the Manchurian border. Combining the active-duty strength of the Red Army and border guards with the call-up of the Far East reserves, approximately one in five Soviet soldiers was sent to the frontier, the largest Red Army combat force to be fielded between the Russian Civil War (1917–1922) and the Soviet Union's entry to the Second World War. In 1929, the Chinese Northeastern Army took over the Chinese Eastern Railway to regain solo control of the railway. The Soviet Union quickly responded with a military intervention and eventually forced the Chinese to return the railway to the previous format of joint administration. During the conflict, the Soviets gained full control of Bolshoy Ussuriysky Island () from China, and the military occupation remained on the island until the 1991 Sino-Soviet Border Agreement in which Russia decided to return the western half of the island to China in 2004. Background On 25 July 1919, the Soviet government's Assistant Commissar of Foreign Affairs, Lev Karakhan, had issued a manifesto to the Chinese government that promised the return of the Chinese Eastern Railway (CER) to Chinese control at no financial cost. On 26 August, the Karakhan Manifesto was published by the Soviet press, but the document mentioned neither the return of CER to the Chinese nor the lack of financial compensation. Along with the original Karakhan telegram, the Chinese had the Vilenski pamphlet as evidence. The Vilenski pamphlet also showed the Chinese that the Soviets were willing to transfer the CER to the Chinese without compensation. The July 25 Karakhan telegram showef the Soviet Union's original intention, to return the CER to Chinese control without compensation. The July 25 telegram satisfied the diplomatic requirements for the Chinese government, but the August 26 one was published to uphold Soviet propaganda requirements. The first major step in uncovering the hostile takeover of the CER by the Chinese in 1929 started with the understanding of the Secret Protocol of March 14, 1924 and the Secret Agreement of September