Complex sentence: Three and a half companies remained in each front battalion area for mobile defence; during attacks the reserve battalion was to advance and occupy the Hindenburg Line, such dispositions doubled the area held by a unit compared to July 1916 on the Somme.
Split into simpler sentences: Three and a half companies remained in each front battalion area for mobile defence; during attacks the reserve battalion was to advance and occupy the Hindenburg Line. The new dispositions doubled the area held by a unit compared to July 1916 on the Somme.