Thanks to this slow but continuous advance, and to the capture of these various villages, the fortress of Chaulnes was outflanked and half-encircled. However, the Germans managed to maintain themselves there, and the French progress was held in this sector, as it had been further north, by the stronghold of Barleux and the marshes of the Somme. At the end of 1916, the front line of the sector south of the Somme started from Omiécourt, left Barleux in German hands, and crossed the Maisonnette Plateau. From there, it described a large circle _via_ Berny (French) and Chaulnes (German), skirting Roye and Lassigny _(see sketch map, p. 29)_. The German Retreat of March, 1917 Although the Somme offensive did not give immediate strategical results, it nevertheless procured the Allies tactical advantages which were one of the causes of the German retreat of March, 1917. The capture of important points of support made the position of the Germans a very precarious one, at all the points where they had so far succeeded in maintaining themselves. They feared that if in 1917 the Allies resumed their offensive--which the experience acquired in 1916 would render still more formidable--further retreat, resulting in the piercing of their front line, might become necessary. They consequently decided voluntarily to shorten their lines by falling back on new positions in the rear, known as the "Hindenburg Line" (see the Michelin Guide: +"The Hindenburg Line"+). [Illustration: THE BAND OF THE AUSTRALIAN 5TH BRIGADE PASSING THROUGH THE SMOKING RUINS OF BAPAUME ON MARCH 19, 1917, WHILE THE BATTLE STILL RAGED NEAR BY, ON THE LINE BECQUINCOURT--NOVAINS.] The formation of a new defensive front was only possible by evacuating a large area, and the German retreat extended to the whole of the region comprised between Arras and Soissons. It was very skilfully carried out, unhampered by the Allies, who contented themselves with following close behind the retreating enemy. On March 15 and 16, 1917, the French, informed by their Air Service of the enemy's imminent retirement, made numerous raids into the German trenches between the Oise and the Avre, advancing in places as much as 4 km. On the 17th, the cavalry, followed by the infantry, entered Lassigny and Roye. Noyon was occupied early on the 18th. On the same day (March 17) the British, having relieved the French as far as south of Chaulnes during the winter, captured La Maisonnette, Barleux, Villers-Carbonnel and all the villages still occupied by the enemy within the loop of the Somme. On the 18th, they entered Péronne and Chaulnes.