Complex sentence: The operation, which began during the Johnson administration, resulted in heavy bombing of Cambodia; by one measurement more bombs were dropped over Cambodia than the Allies dropped during World War II. The bombing killed between 40,000 and 150,000 civilians and combatants.
Split into simpler sentences: The operation, which began during the Johnson administration, resulted in heavy bombing of Cambodia. See The bombing killed between 40,000 and 150,000 civilians and combatants.