Complex sentence: The system met with some success and although it was not fully working until late summer 1918 (the last air raid took place on 19 May) the lessons learnt were to provide valuable grounding for later developments.
Split into simpler sentences: The system met with some success and although it was not fully working until late summer 1918 (the last air raid took place on 19 May). The lessons learnt were to provide valuable grounding for later developments.