 Frisia, the most peripheral part of the duchy, fell under the office of Adalbold, bishop of Utrecht, and under Dirk III, to whom the coastal defence in the west had been delegated.Count Dirk had his power base in Vlaardingen, along the banks of the Merwede, where it merged with the Maas River.Emperor Henry II organised his expedition against the Frisians and their count for two reasons.