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276 front lay an enemy whose armies were capable of launching a vigorous counter-offensive. Strong detachments of his infantry and cavalry occupied tactical points along the line of advance, serving to keep the enemy supplied with information and as a screen to his own movements. His guns, which had already been withdrawn to previously prepared positions, were available at any moment to cover and support a sudden counter-stroke, while the conditions of the country across which the Allies were moving made the progress of their own artillery unavoidably slow. The bulk of the enemy's forces were known to be occupying a very formidable defensive system, upon which he could fall back should his counter-stroke fail. On the other hand the Allies, as they moved forward, lft all prepared defences farther and farther behind them. In such circumstances the necessity for caution was obvious. In order to combat these dangers and difficulties, the Allies were compelled, at the various stages of the advance, to select and put into a state of defence successive lines of resistance, which the main bodies would occupy, and in which they could give battle in the event of a hostile riposte. Meanwhile advanced guards, patrols and reconnoitring detachments pushed ahead and maintained touch with the enemy, and roads, railways, bridges, telegraphs and telephones were constructed or restored with rapidity.

As the Allies approached the Hindenburg Line, the resistance of the enemy stiffened and the fighting for the various tactical localities became more severe. During the first week in April the British had almost reached the Hindenburg Line, and had gained possession of the villages of Lempire, Epehy, Metz-en-Couture, Lagnecourt, Noreuil, Croisilles, Henin-sur-Cojeul and Beurains. The enemy was now withdrawn to his great defensive line, which he had taken months to prepare, shortening his front considerably and enabling him to bring several divisions into reserve. He had in front of him a devastated zone where the mounting of an Allied attack would present extreme difficulties, thereby releasing yet other troops from the guard of the trenches to pass into reserve.

(G) (April-May 1917). At the close of 1916, and before the appointment of Gen. Nivelle to the command of the French forces, a general plan had been agreed upon by all the Allies to conduct a simultaneous offensive on all fronts. The British part in this offensive was to consist of a double attack along the Scarpe and Ancre valleys, to cut off the Gommecourt salient created as a result of the Somme fighting. The V. Army was to attack along the Ancre, the III. Army was to debouch from Arras along the Scarpe, while the I. Army (Canadian Corps) was to secure the left flank of these operations by seizing the Vimy ridge. Owing to the exhaustion of the French armies the main operations were to be conducted by the British, who would not press the attack unduly in the direction of Cambrai, but be prepared to switch rapidly to the N. and undertake further operations of greater dimensions in Flanders.

The appointment of Gen. Nivelle in Dec. 1916 to the chief command of the French, and the acceptance of his plan of offensive, dislocated the strategical plans for the British armies at this particular juncture.

The Nivelle plan gave the French the major r&le, as indicated earlier, namely, that of delivering a 'decisive blow from the Aisne front, while the British operations in Artois were to be of a subsidiary character. It is to be noted in this connexion that the extension of the front in relief of the French caused a reduction of power to the British which materially affected their operations throughout the remainder of the year. The Nivelle plan relied on the success of the main offensive by the French, but if that failed the subsequent operations would be seriously handicapped. This was a matter of first-class importance, seeing that the French had already been overstrained while the British were reaching the height of their strength and efficiency. Apart from this, the actual tactical scheme and dispositions required little modification as a result of the acceptance of the Nivelle plan.

General Nivelle did not altogether approve of the plan of the British commander-in-chief, and urged that the attack on the Vimy heights should be abandoned and that the concentration should be effected farther S. on the Arras-Ancre front, pointing out that the inclusion of Vimy would cause too wide an exten. sion and dissipation of force. This point is of some interest as showing the difficulties which a commander has to face in the selection of his front of attack and objectives. Sir Douglas Haig had, however, given the closest attention to the various factors affecting the situation, and refused to give way. His arguments were mainly two: firstly, that the capture of the Vimy ridge was essential to secure the left of his operations, and secondly, that any attack mounted S. of Arras, and S. of the point where the Hindenburg Line hinged on the main German front, would be delivered into a pocket or would be entirely dislocated by a voluntary withdrawal of the enemy from the Gommecourt salient, whereas the German forces were bound to stand and fight for the Vimy ridge. The British commander-in-chief scented the retreat of the Germans to the Hindenburg Line and laid his plans to meet that eventuality. The result was that even after the German retreat little or no alteration was necessary. Had he, however, given way to the pressure placed upon him and mounted his attack from Arras to Gommecourt, the operation, which was required by Gen. Nivelle to draw in the hostile reserves and pave the way for the main French attack on the Aisne, could not have taken place at all. The task of the British was to attract as large forces of the enemy as possible and so reduce the opposition to the French. As soon as the German retreat developed, all those troops and heavy artillery which were not required with the V. Army in its advance from the Ancre were diverted to the III. and I. Armies in order to strengthen their attacks to the utmost.

The preparations for a great offensive, where reliance is placed on artillery to destroy the enemy's defences and reduce his fire-power to such a point as to enable a successful advance to be made, are extremely long and arduous. When transport requirements on the Arras front were first brought under consideration, the neighbourhood was served by two single lines of railway leading to Arras, the combined capacity of which was less than half the requirements. Considerable constructional work, therefore, both of standard- and narrow-gauge railway, had to be undertaken to meet the programme. Roads had to be improved and adapted; new roads had to be constructed, and material massed forward for construction across the enemy's defences as soon as the troops advanced. For this latter purpose use was made both in this and in later offensives of plank roads. These were built chiefly of heavy beech slabs laid side by side, and were found to be of great utility, being capable of rapid construction over almost any nature of ground. By these means the accumulation of the vast stocks of munitions and stores of all kinds required for the offensive, and their distribution to the troops, were made possible. Hutting and other accommodation for the troops concentrated in the area had to be provided in great quantity; an adequate water-supply had to be guaranteed. Very extensive mining and tunnelling operations were also carried out. In particular, advantage was taken of the existence of a large system of underground quarries and cellars in Arras and its suburbs to provide safe quarters for a great number of troops. Electric light was installed in these caves and cellars, which were linked together by tunnels, and the whole connected by long subways with the trench systems E. of the town.

A problem peculiar to the launching of a great offensive from a town arose from the difficulty of ensuring the punctual debouching of troops and the avoidance of confusion and congestion in the streets both before the assault and during the progress of the battle. This problem was met by the most careful and complete organization of routes.

Practically the labour of the whole of the troops was required to carry through all this work, and while this immense task was proceeding, the British IV. and V. Armies were vigorously pursuing the enemy to the Hindenburg Line, and the French front was in process of being relieved as far S. as the Amiens-Roye road. All this placed a heavy strain on the British troops.