Complex sentence: By AD 900 the Vikings secured for themselves a foothold on Frankish soil along the Lower Seine River valley in what is now France, thereafter establishing the Duchy of Normandy, a territorial acquisition which provided them the opportunity to expand beyond Normandy into Anglo-Saxon England.
Split into simpler sentences: By AD 900 the Vikings secured for themselves a foothold on Frankish soil along the Lower Seine River valley in what is now France that became known as Normandy. They established the Duchy of Normandy, a territorial acquisition which provided them the opportunity to expand beyond Normandy into Anglo-Saxon England.