Complex sentence: Casualties had at one stage reduced the battalion to 200--300 all ranks, around a quarter of its original strength; other battalions fared little better, 3rd Brigade having been reduced to little more than battalion strength.
Split into simpler sentences: Casualties had at one stage reduced the battalion to 200--300 all ranks, around a quarter of its original strength. Other units fared little better, with 3rd Brigade having been reduced to little more than battalion strength.