Complex sentence: Fraser was then re-employed by the British Army in the rank of colonel from 18 December 1941 until November 1945 as the military attaché in Teheran; he was restored to the rank of major-general on retiring once again.
Split into simpler sentences: Fraser was then re-employed by the British Army in the rank of colonel from 18 December 1941 until November 1945 as the Military Attaché, Teheran. He was restored to the rank of major-general on retiring once again.