World War I broke out when Austria-Hungary invaded Serbia on 28 July 1914. Russia entered the war on 1 August 1914. On 22 August 1914, Sheydeman's 2nd Army Corps was transferred to the 1st Army under General Paul von Rennenkampf. After General Samsonov's 2nd Army was almost completed destroyed during the Russian defeat in the Battle of Tannenberg in late August and an ashamed Samsonov committed suicide, Sheydeman took command of the 2nd Army on 5 September 1914. In September, Sheydeman's 2nd Army participated in the Russian offensive in East Prussia. Although the 2nd Army delivered a blow from the south that forced the Imperial German Army to divert some forces to face it and saved the Russian 1st Army from destruction, the Russian offensive ended in a Russian defeat.