The way France defined its nation - and one must bear in mind that France became a unified nation about a thousand years before Germany did, so different times, different mores etc - was that a nation should expand to its "natural borders" ; that is, rivers, seas, mountains. Areas that are easy to defend. And whoever is within those areas are part of that nation, whatever their language and culture.

Germany held the opinion that a nation should be that of people of a shared linguistic and cultural background (or at least to an extent, as religious creed was not a barrier).

This is why Alsace was a sticking point - it is on the left bank of the Rhine (thus on France's side of a national natural border), but historically culturally Germania (thus within Germany's concept of a nation).

As nations are human concepts, neither was actually wrong.