Complex sentence: The Germans had moved up their reserve units on the previous night in anticipation of an attack and the main assembly of Canadian troops was detected by and within three minutes of the attack commencing, the German artillery brought down defensive fire at widely scattered points.
Split into simpler sentences: The Germans had moved up their reserve units on the previous night in anticipation of an attack. The main assembly of Canadian troops was detected by and within three minutes of the attack commencing the German artillery brought down defensive fire at widely scattered points.