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. In September 1940, he was promoted to lieutenant-colonel and assigned to
OKW. 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 10 September 1943, he was awarded the
Knight's Cross of the Iron Cross. 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 26 June 1944, he was awarded the
Knight's Cross of the Iron Cross with Oak Leaves. On 11 February, he was promoted to command the
III SS Panzer Corps and remained with the Corps until 5 March. On 4 April he was appointed to command the newly formed
Panzer Division Clausewitz. ==Awards and decorations==