The Invisible Flag by Peter Bamm. 
From 1941 until 1945 Peter Bamm served as a surgeon with the German Army on the Russian front. He was operating on casualties, often under fire, from the crossing of the Dniester almost to the Caspian Sea and back to the Crimea.
This German best-seller is an eye-witness account of the titanic struggle between monster armies in the fly-ridden Russian summers and the mud and cold of Russian winters. One is struck by the tireless efficiency of the Wehrmacht in advance and retreat, by its uneasy relations with the Nazi Party, and by those crashing political blunders which antagonized hordes of Russians who would willingly have helped to oust the Communist regime.
But principally one's heart must be touched by this human and humorous book as it records the feats of brave men banded together under 'the invisible flag- the flag of humanity'.