Complex sentence: The 6th Army commanders had also been reluctant to encourage the British to change their plans, if they detected a thinning of the front line and were inhibited by the extent of British air reconnaissance, which observed new field works and promptly directed artillery fire on them.
Split into simpler sentences: The 6th Army commanders had also been reluctant to encourage the British to change their plans, if they detected a thinning of the front line. The commanders were inhibited by the extent of British air reconnaissance, which observed new field works and promptly directed artillery fire on them.