Complex sentence: The main assault fell on John's Ridge, which was being held by an understrength platoon, had to be reinforced, after which, amidst heavy rain, the Australians counter-attacked with two platoons hitting the Japanese flanks.
Split into simpler sentences: The main assault fell on John's Ridge, which was being held by an understrength platoon, had to be reinforced. Amidst heavy rain, the Australians counter-attacked with two platoons hitting the Japanese flanks.