Complex sentence: From the summer of 1940, he fought with his unit in France and was moved in 1941 to the eastern front, in the following winter because of the illness of double pneumonia he was transferred back to Germany and was used as an accountant.
Split into simpler sentences: From the summer of 1940, he fought with his unit in France and was moved in 1941 to the eastern front. In the following winter, because he had contracted double pneumonia, he was transferred back to Germany and was used as an accountant.