Labour was transferred south in February 1917 to work on the Hundingstellung from La Fère to Rethel and on the forward positions on the Aisne front, which the Germans knew were due to be attacked by the French.

Movement behind the German front was made difficult by constant Anglo-French artillery harassing-fire, which added to equipment shortages by delaying deliveries by rail and interrupting road maintenance.

Generalleutnant Georg Fuchs, one of the corps commanders, recommended that a defensive line be built from Arras to west of Laon, shortening the front by 25 mi (40 km) and releasing ten divisions which, with other troops, could be used for an offensive in Alsace or Lorraine.

Ernst von Wrisberg (de) Deputy Minister of the Prussian Ministry of War, responsible for raising new units, had grave doubts about the wisdom of this increase in the army but was over-ruled by Ludendorff.

The Battle of the Somme further reduced the German reserve of ammunition and when the infantry was forced out of the front position, the need for Sperrfeuer (defensive barrages), to compensate for the lack of obstacles, increased.

The Siegfriedstellung (Siegfried Position, known to the British as the Hindenburg Line) was to be built across the base of the Noyon Salient, from Neuville Vitasse near Arras, through St Quentin and Laon, the Aisne east of Soissons to Cerny en Laonnois on the Chemin des Dames ridge.

Transport of materials was conducted by canal barge and railway, which carried 1,250 trainloads of engineering stores, although the building period from October 1916 to March 1917 meant that only about eight trains a day were added to normal traffic.

Where the lay of the land gave the defender a tactical advantage, by which an attacker could be defeated with the minimum of casualties to the defenders, with small-arms fire from dispersed, disguised positions and observed artillery-fire, it was to be fought for by the garrison and local reserves, which would counter-attack to regain any ground lost The changes were codified in a training manual Grundsätze für die Führung in der Abwehrschlacht (The Conduct of the Defensive Battle in Position Warfare) issued on 1 December 1916, which made infantry sections (Gruppen) rather than the battalion the basic tactical unit.

Two schools of thought emerged over the winter; the principal authors of the new training manual, Colonel Max Bauer and Captain Hermann Geyer of the General Staff, wanting front garrisons to have discretion to move forwards, sideways and to retire.

Existing operations were to continue over the winter, fresh troops arriving in front-line units were to be trained and in the spring the front of attack was to be broadened, from the Somme to Arras and the Oise.

Three intermediate defensive lines begun in late 1916, much closer to the Somme front, were observed by British reconnaissance aircraft, which made fragmentary reports of digging further back unexceptional.

In February, attempts to send more aircraft to reconnoitre the line were hampered by mist, snow, rain, low cloud and an extremely determined German air defence.

On the night of 12 March, the Germans withdrew from the Riegel I Stellung between Bapaume and Achiet le Petit, while small parties of troops sent up flares to mislead the British, who were preparing an attack.

[48] On 24 February Lieutenant-General Hubert Gough defined the boundaries of the three corps making the advance and ordered them to regain contact with the German armies, using strong patrols supported by larger forces moving forward more deliberately behind them.

Fires could be seen behind Bapaume, with more visible behind the Riegel III Stellung and British military intelligence reported that the headquarters of Rupprecht had been moved to Mons; civilians were known to have been evacuated along with supply dumps and artillery.

[51] The 1st Army from Arras to Péronne brought reserve Siegfried divisions forward to the Riegel III Stellung and outpost villages close to the Siegfriedstellung (Hindenburg Line).

The bridges over the rivers Germaine, Omignon, Cologne, Tortille and the Canal du Nord were also destroyed and huge craters blown in crossroads, the damage being made worse by the spring thaw.

[55] Outpost villages close to the Siegfriedstellung (Hindenburg Line) south of Quéant had to be held by the Germans for longer than expected, because of the need to complete the additions to the defences being built to remedy defects in the original position.

On 15 March the French Groupe d'armées du Nord (GAN), south of the junction with the British Fourth Army at Roye, was ordered to follow up a German retirement.

[61] After 18 March the main body of the Fifth Army was ordered to dig in temporarily from Bancourt to Bapaume, Achiet-le-Grand and Ablainzevelle and the advanced guards, which were large enough to be mobile columns, be reinforced to the strength of brigade groups.

Next day the brigade groups were to support the cavalry drive the Germans back to the Hindenburg Line, which led the 2nd Australian Division force to attack Noreuil on 20 March.

By late March each British corps in the pursuit had diverted a minimum of one division to work on road repairs and bridging, the thaw making the effect of German demolitions far worse.

Information from captured documents and prisoners had disclosed the details of Unternehmen Alberich and that outpost villages had to be held for longer than planned, to enable work to continue on the Hindenburg Line (Siegfriedstellung), where it was being rebuilt south of Quéant.

Despite increased German resistance, Neuville Bourjonval, Ruyaulcourt, Sorel le Grand, Heudicourt, Fins, Dessart Wood, St Emilie, Vermand sur Omignon, Vendelles, Jeancourt, Herbecourt, Épehy and Pezières were captured between 28 March and 1 April.

Pilots flew low over villages and strong points to invite German ground fire for their observers to plot, although this practice gave no indication of the strength of rearguards.

[74] Belated awareness of the significance of the building work along the base of the Noyon Salient, has also been given as a reason for a cautious pursuit deliberately chosen, rather than an inept and failed attempt to intercept the German retirement.

[75] In Cavalry Studies: Strategical and Tactical (1907) Haig had described the hasty retreat of a beaten enemy and an organised withdrawal by a formidable force, capable of rapidly returning to the attack, to defeat a disorganised pursuit.

[79] Defending villages as outposts, with most of the rearguard posted at the western exits, left them vulnerable to encirclement and attacks from commanding ground and the predictability of such methods, provided French and British troops with obvious objectives.

Allied troops in the pursuit suffered from exposure and shortages of supplies but had increased morale, better health (trench foot cases declined sharply) and adapted to open warfare.

The British sent 378 tanks to roll through the Siegfriedstellung (Hindenburg Line) barbed-wire fields, as a substitute for a long wire-cutting bombardment and the ground assault was accompanied by a large number of ground-attack aircraft.