On 1 August 1914, at the outbreak of the First World War, Below was given command of I Reserve Corps as part of 8th Army on the Eastern Front. He led his Corps in the Battles of Gumbinnen, Tannenberg and the Masurian Lakes. As a result of his successes, he was promoted to "General der Infanterie" at the end of August 1914 and to command of 8th Army at the beginning of November.