Complex sentence: As a result of his convalescence, Captain Clark was transferred to the General Staff Headquarters of the First United States Army until the end of hostilities, then served with the Third Army in its occupation duties in Germany.
Split into simpler sentences: As a result of his convalescence, Captain Clark was transferred to the General Staff Headquarters of the First United States Army until the end of hostilities. He then served with the Third Army in its occupation duties in Germany.