Let’s take a look at the basic political geography of the Franks.
Two kingdoms made up the core of the Frankish lands, Neustria in the west and Austrasia in the east. They can be very roughly compared to France and Germany, with many caveats. We’ll start with Austrasia, traditionally the larger, stronger of the two, and the homeland of both the Merovingian and Carolingian dynasties.
Austrasia occupied the drainage area of the lower Rhine, including the Main and the Meuse rivers. The Franks did not, however, control the mouths of the Rhine and the Meuse, rather leaving those under control of the Frisians. They did control the coast south of the Meuse, down to approximately the Franco-German border today. When you think of Austrasia, think of Belgium, Luxembourg, the eastward part of the Netherlands, and north-western Germany. The countryside tends to open and flat. The German army took this path in the opening blow of the First World War in their march on Paris (the infamous “right wing”).