It was at this period that a great change came over both the object and the locality of the operations. This change depended upon two events which had occurred far to the north, and reacted upon the great armies locked in the long grapple of the Aisne. The first of these controlling circumstances was that, by the movement of the old troops and the addition of new ones, each army had sought to turn the flank of the other in the north, until the whole centre of gravity of the war was transferred to that region. A new French army under General Castelnau, whose fine defence of Nancy had put him in the front of French leaders, had appeared on the extreme left wing of the Allies, only to be countered by fresh bodies of Germans, until the ever-extending line lengthened out to the manufacturing districts of Lens and Lille, where amid pit-shafts and slag-heaps the cavalry of the French and the Germans tried desperately to get round each other's flank. The other factor was the fall of Antwerp, which had released very large bodies of Germans, who were flooding over Western Belgium, and, with the help of great new levies from Germany, carrying the war to the sand-dunes of the coast. The operations which brought about this great change open up a new chapter in the history of the war. The actual events which culminated in the fall of Antwerp may be very briefly handled, since, important as they were, they were not primarily part of the British task, and hence hardly come within the scope of this narrative.