As at Verdun, but now on a huge scale, Nivelle trained troops to carry out his plans to a glittering conclusion. The battering force of French artillery shells would move forward rapidly ahead of the troops in a "rolling barrage." The infantry would follow with equal speed, with fresh troops brought up continually to move through the positions taken by the first waves. As the French forces concentrated, the elements that destined them to failure began to appear. Through a variety of sources—spies, captured field orders, but also just by observing the masses of French forces—the Germans learned Nivelle's intentions. They responded with a defensive measure just as novel as Nivelle's plan of attack: they withdrew from their salient. Destroying everything behind them and leaving thousands of booby traps, they pulled back between March 15 and 19. The vulnerable salient no longer existed. Ludendorff had reduced the length of his front by almost thirty miles. Thirteen divisions had been removed from the front line to constitute a potent reserve. Beyond these measures, the Germans embarked on a sophisticated reconstruction of their battle line aimed at multiple networks of defensive lines and groups of strongpoints. Mobile defensive forces would be able to avoid enemy artillery fire, abandon unpromising sectors of the line, and then counterattack to retake crucial areas. As this formidable system was taking shape, Nivelle prepared his attack for early April. Despite the renewed strength of the German defenses, the need for a victorious offensive persisted, and Nivelle promised one. The attack would go ahead as planned. In the north, the British were to make their diversionary assault eastward. The main thrust was still to take place in the south: even after thd German withdrawal, part of their line still rested along the Aisne River. Here the French would carry out Nivelle's design.