Complex sentence: A pause in the EEF's advance was necessary to enable the lines of communication to be lengthened and strengthened, and while this work was being carried out, the mounted brigades were reorganised into two mounted divisions.
Split into simpler sentences: A pause in the EEF's advance was necessary to enable the lines of communication to be lengthened and strengthened. While this work was being carried out, the mounted brigades were reorganised into two mounted divisions.