Complex sentence: The information that he leaked is believed to have been potentially among the more damaging disclosures made by a Second World War spy but conflicts inside the highest echelons of the German government meant that little if any of it was acted upon.
Split into simpler sentences: The information that he leaked is believed to have been among the potentially more damaging disclosures made by a Second World War agent. Distrust inside the highest echelons of the German government meant that little, if any, of it was acted upon.