Complex sentence: Two officers were later executed on war crimes charges for their involvement, although the historicity of the event has been hotly contested, often by Japanese nationalist or revisionist historians seeking to invalidate the historiography of the Nanking Massacre.
Split into simpler sentences: Two officers were later executed on war crimes charges for their involvement. Since that time, the historicity of the event has been hotly contested, often by Japanese nationalist or revisionist historians seeking to invalidate the historiography of the Nanking Massacre.