Complex sentence: Hitler and the German commanders who fought at Crete were shocked by the very high casualties of the paratroopers sustained in the capture of the island and as a result, the Germans were forced to reconsider their airborne doctrine.
Split into simpler sentences: Hitler and the German commanders who had fought on Crete were shocked by the very high casualties of the paratroopers sustained in the capture of the island. As a result, the Germans were forced to reconsider their airborne doctrine.