Complex sentence: He was the Commander-in-Chief of the Allied ground forces in Central Europe from April 1957 to September 1963, an impressive achievement considering he had been a General under Hitler fifteen years before.
Split into simpler sentences: He was the Commander-in-Chief of the Allied ground forces in Central Europe from April 1957 to September 1963. An impressive achievement by any standard, but all the more so when you consider he had been a General under Hitler fifteen years before.