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On 15 September Generalfeldmarschall Crown Prince Rupprecht, commander of the northern group of armies, was ordered to prepare a rear defensive line and on 23 September work on the new Siegfriedstellung (Hindenburg Line) began. On 21 September, after the battle of Flers–Courcelette (15–22 September), Hindenburg ordered that the Somme front was to have priority in the west for troops and supplies. By the end of the Battle of Morval (25–28 September) Rupprecht had no reserves left on the Somme front. During September, the Germans sent another thirteen fresh divisions to the British sector and scraped up troops wherever they could be found. The German artillery fired 213 train-loads of field artillery shells and 217 train-loads of heavy ammunition, yet the début of the tank, the defeat at the Battle of Thiepval (26–28 September) and the number of casualties (September was the costliest month of the battle for the German armies) had been severe blows to German morale.

In early 1916, the German army had 900,000 men in recruit depots and another 300,000 due in March when the 1897 class of conscripts was called up. The army was so flush with men that plans were made to demobilise older Landwehr classes and in the summer, Falkenhayn ordered the raising of another 18 divisions, for an army of 175 divisions. The costly battles at Verdun and the Somme had been much more demanding on German divisions and they had to be relieved after only a few days in the front line, lasting about 14 days on the Somme. A larger number of divisions might reduce the strain on the Westheer and realise a surplus for offensives on other fronts. Hindenburg and Ludendorff ordered the creation of another 22 divisions, to reach 179 divisions by early 1917. The men for the divisions created by Falkenhayn had come from reducing square divisions with four infantry regiments to triangular divisions with three regiments, rather than a net increase in the number of men in the army.

Despite the shortfalls, by the summer of 1917, the Westheer artillery park had increased from 5,300 to 6,700 field guns and from 3,700 to 4,300 heavy guns, many being newer models of superior performance. Machine-gun output enabled each division to have 54 heavy and 108 light machine-guns and for the number of Maschinengewehr-Scharfschützen-Abteilungen (MGA, machine-gun sharpshooter detachments) to be increased. The greater output was insufficient to equip the new divisions, and existing divisions, which still had two artillery brigades with two regiments each, lost a regiment and the brigade headquarters, leaving three regiments. Against the new scales of equipment, British divisions in early 1917 had 64 heavy and 192 light machine-guns and the French 88 heavy and 432 light machine-guns.

Mihiel Salient. The new fortified areas were intended to be precautionary measures (Sicherheitskoeffizient) built to be used as rallying-positions (Eventual-Stellungen, similar to ones built on the Russian front) and to shorten the Western Front to economise on troops and create more reserves. The Siegfriedstellung had the potential to release the greatest number of troops and was begun first; Hindenburg and Ludendorff decided its course on 19 September and construction began on 27 September. Withdrawal to the Siegfriedstellung was debated by Ludendorff and other senior German commanders over the winter of 1916–1917. An offensive in the new year with 21 divisions was discussed on 19 December but it was considered that such a force could not achieve a decisive result. An Oberste Heeresleitung (OHL) memorandum of 5 January, noted that offensive preparations by the French and British were being made all along the Western Front so as to keep the site of a spring offensive secret.

Where the lay of the land gave observation from behind the system, it was built on reverse slopes (a Hinterhangstellung), with a short field of fire for the infantry, according to the experience of the Western Front defensive battles of 1915 and 1916, when forward-slope positions had been smashed by observed Franco-British artillery-fire. In much of the new position, the new principles of reverse-slope positions with artillery-observation posts to the rear was not followed. Artillery observation posts were built in the front-trench system or in front of it. Trenches had been dug near a crest, on a forward slope or at the rear of a reverse slope, which replicated the obsolete positions being abandoned. The 1st Army commander, General von Below, and his Chief of Staff Colonel von Lossberg rejected this layout since smoke and dust would make artillery observation from such positions impossible.

Allgemeines über Stellungsbau (Principles of Field Fortification) was published in January 1917 and by April an outpost zone (Vorpostenfeld) held by sentries, had been built along the Western Front. Sentries could retreat to larger positions (Gruppennester) held by Stosstrupps (five men and an NCO per Trupp), who would join the sentries to recapture sentry-posts by immediate counter-attack. Defensive procedures in the battle zone were similar but with greater numbers. The front trench system was the sentry line for the battle zone garrison, which was allowed to move away from concentrations of enemy fire and then counter-attack to recover the battle and outpost zones; such withdrawals were envisaged as occurring on small parts of the battlefield which had been made untenable by Allied artillery fire, as the prelude to Gegenstoss in der Stellung (immediate counter-attack within the position). Such a decentralised battle by large numbers of small infantry detachments would present the attacker with unforeseen obstructions. Resistance from troops equipped with automatic weapons, supported by observed artillery fire, would increase the further the advance progressed.

Military historians have attributed the success of this attack to careful planning by Canadian Corps commander Julian Byng and his subordinate General Arthur Currie, constant training, and the assignment of specific objectives to each platoon. By giving units specific goals, troops could continue the attack even if their officers were killed or communication broke down, thus bypassing two major problems of combat on the Western Front. After the territorial gains of the first two days, a hiatus followed as the immense logistical support needed to keep armies in the field caught up with the new realities. Battalions of pioneers built temporary roads across the churned up battlefield; heavy artillery (and its ammunition) was manhandled into position in new gun pits; food for the men and feed for the draught horses was brought up, and casualty clearing stations were established in readiness for the inevitable counter-attacks. Allied commanders also faced a dilemma: whether to keep their exhausted divisions on the attack and run the risk of having insufficient manpower or replace them with fresh divisions and lose momentum. In London, The Times commented: "the great value of our recent advance here lies in the fact that we have everywhere driven the enemy from high ground and robbed him of observation.

In early 1918, The Times carried an article, Falkenhausen's Reign of Terror, describing 170 military executions of Belgian civilians since he had been appointed governor. Ludendorff and Lossberg discovered that although the Allies were capable of breaking through the first position they could probably not capitalise on their success if they were confronted by a mobile, clever defence. Ludendorff immediately ordered more training in manoeuvre warfare for his Eingreif divisions. Lossberg was soon promoted to general and directed the defensive battle of the 4th Army against the British Flanders offensive of the summer and late autumn. (Lossberg had become "legendary as the fireman of the Western Front; always sent by OHL to the area of crisis"). Siegfried Sassoon makes reference to the battle in the poem The General The Anglo-Welsh lyric poet Edward Thomas was killed by a shell on April 9, 1917, during the first day of the Easter Offensive. Thomas's war diary gives a vivid and poignant picture of life on the Western front in the months leading up to the battle. The composer Ernest John Moeran was wounded during the attack on Bullecourt on May 3, 1917.

Draught animals suffered from the weather, short rations and overloading; the British artillery soon had a shortage of 3,500 horses and several immobilised heavy artillery batteries. The length of the Western Front was reduced by 25 miles (40 km), which needed 13–14 fewer German divisions to hold. The Allied spring offensive had been forestalled and the subsidiary French attack up the Oise valley negated. The main French breakthrough offensive on the Aisne (the Nivelle Offensive), forced the Germans to withdraw to the Hindenburg Line defences behind the existing front line on the Aisne. German counter-attacks became increasingly costly during the battle; after four days 20,000 prisoners had been taken by the French armies and c. 238,000 casualties were inflicted on German armies opposite the French and Belgian fronts between April and July.

The cost of the Nivelle Offensive in casualties and loss of morale were great but German losses were also high and the tactical success of the offensive in capturing elaborately fortified positions and defeating counter-attacks, reduced German morale. The German had been forced from three of the strongest positions on the Western Front and had failed to recapture them. Vimy Ridge, the Scarpe Heights, the caverns, spurs and plateau of the Chemin des Dames and the Moronvilliers massif had been occupied for more than two years, carefully surveyed by German engineers and fortified to make them impregnable. In six weeks all were lost and the Germans were left clinging to the eastern or northern edges of the ridges of the summits, having lost the western or southern sides.　The French tactic of assault brutal et continu suited the German defensive dispositions, since much of the new construction had taken place on reverse slopes. The speed of attack and the depth of the French objectives meant that there was no time to establish artillery observation posts overlooking the Ailette valley, in the areas where French infantry had reached the ridge. The tunnels and caves under the ridge nullified the destructive effect of the French artillery, which was also inhibited by poor weather, which reduced observation and by German air superiority, which made French artillery-observation aircraft even less effective.

From spring 1916, the British had deployed five tunnelling companies along the Vimy Ridge and during the first two months of their tenure of the area, 70 mines were fired, mostly by the Germans. Between October 1915 and April 1917 an estimated 150 French, British and German charges were fired in this 7 kilometres (4.3 mi) sector of the Western Front. In May 1916, Operation Schleswig-Holstein, a German infantry attack, forced the British back 640 metres (700 yd), to stop British mining by capturing the shaft entrances. From June 1916, the Germans withdrew many miners to work in coal mines in Germany. In the second half of 1916, the British constructed strong defensive underground positions and from August 1916, the Royal Engineers developed a mining scheme for a big infantry attack on the Vimy Ridge proposed for autumn 1916, although this was postponed. After September 1916, when the Royal Engineers had completed their network of defensive galleries along most of the front line, offensive mining largely ceased although activities continued until 1917. The British gallery network beneath Vimy Ridge eventually grew to a length of 12 kilometres (7.5 mi). The Canadian Corps was posted to the northern part of Vimy Ridge in October 1916 and preparations for an attack were revived in February 1917. British tunnelling companies created extensive underground networks and fortifications.

Twelve subways, up to 1.2 kilometres (0.75 mi) long were excavated at a depth of 10 metres (33 ft) and used to connect reserve lines to front lines, permitting soldiers to advance to the front quickly, securely and unseen. Often incorporated into subways were light rail lines, hospitals, command posts, water reservoirs, ammunition stores, mortar and machine gun posts and communication centres. The Germans dug a number of similar tunnels on the Vimy front, to provide covered routes to the front line and protection for headquarters, resting personnel, equipment and ammunition.The Germans also conducted counter-mining against the British tunnellers and destroyed a number of British attempts to plant mines under or near their lines. Prior to the Battle of Vimy Ridge, the British tunnelling companies also secretly laid 13 mines under German positions to destroy surface fortifications before the assault. To protect some advancing troops from German machine-gun fire, as they crossed no man's land during the attack, eight smaller Wombat charges were laid at the end of the subways, to allow troops to move more quickly and safely enter the German trench system, by creating an elongated trench-depth crater that spanned the length of no man's land. At the same time, 19 crater groups existed along this section of the Western Front, each with several large craters.

Allgemeines über Stellungsbau (Principles of Field Fortification) was published in January 1917 and by April an outpost zone (Vorpostenfeld) held by sentries, had been built along the Western Front. Sentries could retreat to larger positions (Gruppennester) held by Stosstrupps (five men and an NCO per Trupp), who would join the sentries to recapture sentry-posts by immediate counter-attack. Defensive procedures in the battle zone were similar but with greater numbers. The front trench system was the sentry line for the battle zone garrison, which was allowed to move away from concentrations of enemy fire and then counter-attack to recover the battle and outpost zones; such withdrawals were envisaged as occurring on small parts of the battlefield which had been made untenable by Allied artillery fire, as the prelude to Gegenstoss in der Stellung (immediate counter-attack within the position). Such a decentralised battle by large numbers of small infantry detachments would present the attacker with unforeseen obstructions. Resistance from troops equipped with automatic weapons, supported by observed artillery fire, would increase the further the advance progressed. A school was opened in January 1917 to teach infantry commanders the new methods.

And Dobell wrote, This action has had the result of bringing the enemy to battle, and he will now undoubtedly stand with all his available force in order to fight us when we are prepared to attack. It has also given our troops an opportunity of displaying the splendid fighting qualities they possess. So far as all ranks of the troops engaged were concerned, it was a brilliant victory, and had the early part of the day been normal victory would have been secured. Two more hours of daylight would have sufficed to finish the work the troops so magnificently executed after a period of severe hardship and long marches, and in the face of most stubborn resistance. — General Dobell, Eastern Force The British press reported the battle as a success, but an Ottoman plane dropped a message that said, "You beat us at communiqués, but we beat you at Gaza."Dallas, the commander of the 53rd (Welsh) Division, resigned after the battle, owing to a "breakdown in health." Judged by Western Front standards, the defeat was small and not very costly. Murray's offensive power had not been greatly affected and preparations for a renewal of the offensive were quickly begun. The Second Battle of Gaza began on 17 April 1917.

The Arras sector then returned to the stalemate that typified most of the war on the Western Front, except for attacks around Lens, culminating in the Canadian Battle of Hill 70 in August.At the beginning of 1917, the British and French were still searching for a way to achieve a strategic breakthrough on the Western Front. The previous year had been marked by the costly success of the Franco–British offensive astride the river Somme, while the French had been unable to take the initiative because of intense German pressure at Verdun until after August 1916. Both battles consumed enormous quantities of resources while achieving virtually no strategic gains on the battlefield. Nonetheless, the cost to Germany of containing the Anglo-French attacks had been high, and given that the material preponderance of the Entente and its allies could only be expected to increase in 1917, Hindenburg and Ludendorff decided on a defensive strategy on the Western Front for that year. This impasse reinforced the French and British commanders' belief that to end the stalemate they needed a breakthrough; while this desire may have been the main impetus behind the offensive, the timing and location were heavily influenced by a number of political and tactical factors.

It was agreed in the London Convention of 16 January, that the French assault on the Aisne would begin in mid-April and that the British would make a diversionary attack in the Arras sector approximately one week prior. Three armies of Field Marshal Sir Douglas Haig, the commander of the British Expeditionary Force (BEF) were in the Arras sector, the Fifth Army (General Hubert Gough) in the south, the Third Army (General Edmund Allenby) in the centre and the First Army (General Henry Horne) in the north and the plan was devised by Allenby. In December 1916, the training manual SS 135 replaced SS 109 of 8 May 1916 and marked a significant step in the evolution of the British Expeditionary Force (BEF) into a homogeneous force, well adapted to its role on the Western Front. The duties of army, corps and divisions in planning attacks were standardised. Armies were to devise the plan and the principles of the artillery component. Corps were to allot tasks to divisions, which would then select objectives and devise infantry plans subject to corps approval.

By the end of the Third Battle of Ypres in November 1917, the effectiveness of the methods of defence introduced in 1917 had been eroded and continuation of a defensive strategy in the west was made impossible. The defeat of Russia gave the German leadership a final opportunity to avoid defeat, rather than the attempts to compete with Allied numerical and industrial superiority, through economic warfare in the Atlantic and the domestic initiatives of the Hindenburg Programme, the Auxiliary Service Law and temporary demobilisation of skilled workers from the army. The accuracy of Great War casualty statistics is disputed. Casualty data available refer to Western Front totals as shown in Winston Churchill's The World Crisis (1923–29) and do not refer directly to the German withdrawal to the Hindenburg Line (Siegfriedstellung) or losses which would be considered "normal wastage", occurring as a consequence of the existence of the Western Front, rather than to particular military operations.

As the Allied operations in the Middle East were secondary to the Western Front campaign, reinforcements requested by General Sir Archibald Murray, commander of the Egyptian Expeditionary Force (EEF), were denied. Further, on 11 January 1917, the War Cabinet informed Murray that large scale operations in Palestine were to be deferred until September, and he was informed by Field Marshal William Robertson, the Chief of the Imperial General Staff , that he should be ready to send possibly two infantry divisions to France. One week later, Murray received a request for the first infantry division and dispatched the 42nd (East Lancashire) Division. He was assured that none of his mounted units would be transferred from the EEF, and was told "that there was no intention of curtailing such activities as he considered justified by his resources." Murray repeated his estimate that five infantry divisions, in addition to the mounted units, were needed for offensive operations.

These guns were supplied with 500 rounds per 60-pdr and 6-inch howitzers, 400 rounds per 8-inch howitzer, 600 rounds per 4.5-inch howitzer, and 600 rounds per 18-pdr. Divisional commanders controlled the use of their divisional artilleries, excepting three brigades of 18-pdrs. The 263rd Brigade, 52nd (Lowland) Division was attached to 54th (East Anglian) Division until 07:30 when it transferred to the 74th (Yeomanry) Division with the objective of defending the Sheikh Abbas Ridge. The 267th Brigade, 53rd (Welsh) Division was attached to the 52nd (Lowland) Division until 07:30 when it returned to the 53rd (Welsh) Division. The 272nd Brigade, 54th (East Anglian) Division was attached to Eastern Force artillery until 18:30 18 April when it came under orders of 74th (Yeomanry) Division. The available guns had been deployed at a ratio of one gun every 100 yards (91 m), compared with one gun every 36 feet (11 m) at Arras, on the Western Front, in April 1917. There were not enough guns to cover the 15,000 yards (14,000 m) front. The light field guns could not produce a sufficiently dense bombardment, and the thin barrage was not effectively strengthened by the naval guns. It quickly became clear that the gunfire from the warships, heavy artillery, and field howitzers, including the gas shells, had not silenced the defenders' artillery. Ten minutes before the infantry attack, 18-pdrs began their covering fire.

The Battle of Pilckem Ridge, 31 July – 2 August 1917, was the opening attack of the Third Battle of Ypres in the First World War. The British Fifth Army, Second Army and the French First Army on the northern flank, attacked the German 4th Army which defended the Western Front from Lille, to the Ypres Salient in Belgium and on to the North Sea coast. On 31 July, the Anglo-French armies captured Pilckem (Flemish: Pilkem) Ridge and areas either side, the French attack being a great success. After several weeks of changeable weather, heavy rain fell during the afternoon of 31 July. British observers in the XIX Corps area in the centre, lost sight of the troops that had advanced to the main objective at the green line and three reserve brigades pressing on towards the red line. The weather changed just as German regiments from specialist counter-attack Eingreif divisions intervened. The reserve brigades were forced back through the green line to the intermediate black line, which the British artillery-observers could still see and the German counter-attack was stopped by massed artillery and small-arms fire. The attack had mixed results; a substantial amount of ground was captured by the British and French, except on the Gheluvelt Plateau on the right flank, where only the blue line (first objective) and part of the black line (second objective) were captured. A large number of casualties were inflicted on the German defenders, 5,626 German prisoners were taken and the German Eingreif divisions managed to recapture some ground from the Ypres–Roulers railway, northwards to St. Julien. For the next few days, both sides made local attacks to improve their positions, much hampered by the wet weather. The rains had a serious effect on operations in August, causing more problems for the British and French, who were advancing into the area devastated by artillery fire and partly flooded by the unseasonable rain. A local British attack on the Gheluvelt Plateau was postponed because of the weather until 10 August and the second big general attack due on 4 August, could not begin until 16 August. Pilckem Ridge　ピルケム高地