LYS, BATTLE OF THE. In the general scheme of the great German offensive of 1918, the idea of breaking through the British-Portuguese front in French Flanders was considered by the German High Command. Although this scheme became secondary to the great offensive further south, the preparations for i± were carried out, firstly in order to mislead the British and Allied headquarters, and secondly that the Germans might be able to revert quickly to it in case the Somme offensive should come to a standstill.
When on March 3o it became obvious that no further progress was possible in the southern battle area, the German Supreme Command decided to put into effect the Flanders attack, the preparations for which were nearly complete. Although they did not anticipate that they would be able to force a decision in this area, there were certain definite advantages to be obtained from a successful blow in the direction of St. Omer-Hazebrouck and the local situation favoured, in their opinion, the prospect of success. This was to be the second blow struck against the British army, a great proportion of which had already been employed on the southern front.
The abnormally dry weather gave good grounds for hope that the Lys valley would be practicable, a state of affairs which was not the case till much later in the spring in a normal year. The only lateral lines of communication in this sector of the British zone, other than the coast railway, ran through St. Pol-Lillers Hazebrouck, and if the use of this could be denied to the British, problems of supply and reinforcement would be immeasurably complicated. It was above all important, in the view of the Ger man Supreme Command, to retain the initiative, and to strike a second blow before the French could organize a counter-attack against the somewhat vulnerable southern flank of the salient which had been created by the success of the Somme offensive.
between Armentieres and the La Bassee canal, in the direction of Hazebrouck. If the VI. Army's attack met with sufficient success, the IV. Army under Arnim was to advance north of Armentieres, which was to be made to fall by envelopment, with the eventual object of securing the heights stretching from Kemmel to Mt. des Cats, south of the Ypres-Poperinghe road. If this were achieved the British and Belgian positions in the Ypres salient and on the Yser would be threatened from the rear. The exten sion of the attack southwards was a secondary consideration, but it was hoped that at least the destruction of the important min ing area south of Bethune might be achieved.
The forces available for the operation of the German VI. Army were four corps, from north to south, the Bavarian II. and German XIX., LV. and IV., to which were allotted nine divisions in the first line and five in corps reserve. Three more divisions were retained in army reserve. The subsidiary attack of the German IV. Army was to be delivered by the X. and XVIII. Res. Corps, with three divisions in the first and one in the second line.
The initial blow fell on the left of the I. and right of the II. British Armies, under Sir Henry Horne and Sir Herbert Plumer, the point of junction between which was, on April 9, the Lys, east of Armentieres. The XI. Corps (British 55th and Portu guese 2nd Divs.) and the XV. Corps (4oth and 34th Divs.) held the front from the La Bassee canal to the Lys, while the IX. Corps (25th, 19th and 9th Divs.) continued the line from the Lys to the Ypres-Comines canal. With the exception of the 55th, all these British divisions had already taken part in the southern battle and had been rapidly made up with reinforcements from home. Three more battle-worn divisions were in corps reserve. The 1st Portuguese division had been withdrawn from the line on April 5 and the 2nd was also to be relieved on April 9, but, pending their relief, it had taken over the whole corps sector.