Complex sentence: Should part of the front line be lost, it was to be retaken by counter-attack and behind the line, new defences were to be built and connected by communication trenches, to delay a further attack, until reserves could be assembled for a counter-attack.
Split into simpler sentences: Should part of the front line be lost, it was to be retaken by counter-attack. Behind the line, new defences were to be built and connected by communication trenches, to delay a further attack, until reserves could be assembled for a counter-attack.