Complex sentence: He was Commander-in-Chief of Middle East Land Forces from 1950 to 1953, when he retired from military service for the second time to become Chairman of the British Transport Commission, a post he held until 1961.
Split into simpler sentences: He was Commander-in-Chief of Middle East Land Forces from 1950 to 1953. He retired from military service for the second time in 1953 to become Chairman of the British Transport Commission, a post he held until 1961.