The Rhine Raids had all but crippled the Frankish Empire in the German areas. With the Frankish Navy faltering and the Frankish armies losing battle after battle, the late 840s were grim for the Franks. The Vikings relied on hit-and-run attacks to keep the Franks on the run. Cities, towns, and other settlements along the Rhine River were sacked by the Vikings. Utrecht and Cologne were among the prosperous cities that had been sacked by the Vikings.