Complex sentence: In 776 the Saxons promised to convert to Christianity and vow loyalty to the king, but, during Charlemagne's campaign in Hispania (778), the Saxons advanced to Deutz on the Rhine and plundered along the river.
Split into simpler sentences: In 776 the Saxons promised to convert to Christianity and vow loyalty to the king, but once Charlemagne went to Spain, they staged further attacks in 778. During Charlemagne's campaign in Hispania (778), the Saxons advanced to Deutz on the Rhine and plundered along the river.