Unrein enlisted the German army in the final stages of World War I and then joined the re-established Reichswehr, where he held various regimental posts through the 1930s.

On 15 September 1941, he was appointed to command a motorcycle battalion in the 6th Panzer Division.

The battalion was almost destroyed fighting outside Moscow, and Unrein was court-martialled, but was absolved of all blame.

On 29 October of that year, he was appointed to command the 14th Panzer Division, which was shortly to be sent to the Eastern Front.

On 4 April he was appointed to command the newly formed Panzer Division Clausewitz.