Complex sentence: Fincham is furious as he thought that Oriani has betrayed them, but when the POWs are put on the train, they find a severely battered Oriani in the prisoner carriage.
Split into simpler sentences: Fincham is furious as he thought that Oriani had betrayed them. However, when the POWs are put on the train, they find a severely battered Oriani in the prisoner carriage.