Complex sentence: Those not involved in fighting or occupation duties were demobilised, the demobilisation of 4,000,000 men which followed the end of the war had within a year reduced the army to 800,000 and by November 1920, to 370,000 men.
Split into simpler sentences: Those not involved in fighting or occupation duties were demobilised. The demobilisation of 4,000,000 men that followed the end of the war had, within a year, reduced the army to 800,000; and by November 1920, this figure had fallen to 370,000 men.