Complex sentence: In the II Corps area to the north, the advance by the 1st Division was foiled when one of the two attacking battalions diverged and a German counter-attack got between the battalions and forced them back to their start lines.
Split into simpler sentences: In the II Corps area to the north, the advance by the 1st Division was foiled, when one of its two attacking battalions diverged. A German counter-attack got between the battalions and forced them back to their start lines.