Complex sentence: The attacking force would pass along a ridge between "B" and "C" positions leaving sufficient troops to contain these German and Ottoman soldiers and with "A" captured "B" and "C" positions would be overlooked and become untenable.
Split into simpler sentences: The attacking force would pass along a ridge between "B" and "C" positions leaving sufficient troops to contain these. Once position "A" was captured "B" and "C" positions would be overlooked and become untenable.