Demme joined the army reserve on 15 September 1914 as a volunteer, serving in World War I as a combat engineer, retiring from active service in December 1918.

Some 18 years later, on 5 January 1937, Demme reactivated in Reserve Service of the reincarnated German Army but he became an active officer on 1 November 1941 in the rank of the Oberstleutnant.

From 1 January – 8 June 1939 he served as an Officer with Special Duties of OKW – Service with the Chief of OKW, Special-Staff W. In the dawn of World War II Demme was transferred to the Staff of Training Leader Aachen 1 (8 June – 26 August 1939) and then to the Staff of the Commander of Fortifications at the Lower Rhine until 10 April 1940.

In following years Rudolf Demme was a commanding officer in 208th Pioneer Replacement Battalion (10 April 1940 – 8 July 1941), 92nd Panzer Pioneer Battalion (8 July – 15 December 1941) and 58th Panzer Pioneer Battalion (15 December 1941 – 15 October 1942), then he was transferred to Revitalization Staff Centre.

On 10 January 1943 Demme became a Commander of the 59th Panzer-Grenadier-Regiment of the 20th Panzer Division and led his unit through the bloody battles on the Eastern Front until 25 July 1944 (Orel, Brjansk, Mogilew, Witebsk, Newel, Bobruisk, Cholm).