Complex sentence: From the onset of the main assault it became apparent that the Belgian army would not be able to hold out for any substantial length of time and tadvance of the Germahe n army through Belgium and France threatened to cut off any escape route from the city.
Split into simpler sentences: From the onset of the main assault it became apparent that the Belgian army would not be able to hold out for any substantial length of time. Moreover, the continuing advance of the German army through Belgium and France threatened to cut off any escape route from the city.