Complex sentence: The disadvantage was that Helles was a long way from the forts that needed to be captured, the Helles forts made up the outer defences of the straits and had been neutralised by naval gunfire and raids by Royal Marines.
Split into simpler sentences: The disadvantage was that Helles was a long way from the forts that needed to be captured. The Helles forts made up the outer defences of the straits and had already been neutralised by naval gunfire and raids by Royal Marines.