Complex sentence: In the south, it was intended to reach the Somme and then hold the line of the river against any French counter-attacks; the southern advance was extended to include an advance across the Somme.
Split into simpler sentences: In the south, it was intended to reach the Somme, and then hold the line of the river against any French counter-attacks. During the planning process, the southern advance was extended to include an advance across the Somme.