Complex sentence: According to the Ras Kassa, his men could no longer stand up to the deadly clouds of mustard gas that the Italians rained down on the roads his troops took, the base camps where they gathered, and any area surrounding them.
Split into simpler sentences: According to the Ras Kassa, his men could no longer stand up to the deadly clouds of mustard gas. The Italians rained down lethal gas on the roads his troops took, the base camps where they gathered, and any area surrounding them.