Complex sentence: The most important such force was the 7,500-strong Sturm-Division "Rhodos" under Generalleutnant Ulrich Kleemann, which was sent to the island of Rhodes.
Split into simpler sentences: The most important German force was the 7,500-strong Sturm-Division "Rhodos" under Lieutenant-General ("Generalleutnant") Ulrich Kleemann. This division was sent to the island of Rhodes.