Complex sentence: Travelling down the Rhine, he landed in what is now the Netherlands where he conquered the Frisians, who thereafter served in his army as allies, and then attacked the Chauci, who lived in northwestern Germany in what is now Lower Saxony.
Split into simpler sentences: Travelling down the Rhine and landing in what is now the Netherlands, he conquered the Frisians, who thereafter served in his army as allies. Then, he attacked the Chauci, who lived in northwestern Germany in what is now Lower Saxony.