The Franco-Prussian War engaged Flach's activities in other directions, and he spent two years (described in his Strasbourg après le bombardment, 1873) at work on the rebuilding of the library and the museum, which had been destroyed by Prussian shells.
Having acted for some time as secretary to Jules Sénard, ex-president of the Constituent Assembly, he published an original paper on artistic copyright, but as soon as possible resumed the history of law.
It explains how the Gallo-Roman villa gave place to the village, with its fortified castle, the residence of the lord; how new towns were formed by the side of old, some of which disappeared; how the townspeople united in corporations; and how the communal bond proved to be a powerful instrument of cohesion.
[3] In these three volumes, which appeared at long intervals, the author's theories are not always in complete harmony, nor are they always presented in a very luminous or coherent manner, but they are marked by originality and vigour.
Flach gave them a solid basis by the wide range of his researches, utilizing charters and cartularies (published and unpublished), chronicles, lives of saints, and even those dangerous guides, the chansons de geste.