Meanwhile de Castelnau had got into position to the north of Lassigny, and fierce fighting was raging about Roye, which was lost and won, taken and retaken. If either side could break through at Roye or Lassigny, it would be able to turn against one or other of the armies to its right or left and roll it up. Neither side, however, could gain and keep a yard of ground at this "death angle." Again the line was extended; by the 30th of September Maud'huy's army was advancing eastwards to the north of the Somme, only to find itself opposed by von Buelow's command. Every new French force brought up to extend the line was met and checked by a corresponding German force. Everywhere the enemy showed wonderful energy. While they were holding Maud'huy's army their troops were being hurried northwards behind their lines, and they were nowhere caught napping. Maud'huy dug himself in on the Albert plateau, and von Buelow could not drive him back. Here, too, there was deadlock.
[Illustration: Sketch Map to illustrate the Extension of the Allied Left.]
Once more the French line was extended farther north, and as it extended so did the corresponding German front. Each side was attempting to outflank the other, and it was clear that the double movements could only be stopped by the sea. So the rival armies went clawing northwards. Between the Oise and Arras the French were holding their own with difficulty; in the Arras position they were fairly strong, but round Lille, which was held by French Territorials, they had but a mere ribbon of troops.
In the closing days of September the French learnt that the Germans had begun a new and very dangerous move. Masses of German cavalry were sweeping across the Belgian flats into France. Uhlans were within sight of the sea, and were threatening Maud'huy's left flank round Lille and among the colliery villages to the north of Arras. There were rumours of many troop trains moving through Liége and Namur and Brussels, and it seemed that the cavalry on Maud'huy's flank were but the vanguard of a huge army which was about to be flung against the French rear.
Nor was this all. The Germans had begun to besiege Antwerp. No one could say how long it could hold out. After the experience of Liége and Namur its chances were small, but it was hoped that the Belgian army might make an obstinate stand outside the circle of forts. No risks, however, could be taken. New armies must be moved without delay to the extreme left of the Allied line, in order to check the new German attack, and also to hold out a helping hand to the defenders of Antwerp.