A week later the anticipated French offensive began, driving northward from the Aisne River, after six days of intensive shelling their infantry was led forward by 128 tanks, the first attack by massed tanks. Nivelle knew that the Germans had captured his detailed plans several weeks before, but followed them nonetheless. The first two German lines were taken at heavy cost and the French slowly advanced as the defense fell back to their main line of resistance it was far from Nivelle's promise of a first day's advance of . The attacks ended in early May when many French regiments refused to attack. The Germans never learned the extent of their enemy's demoralization. Nivelle was replaced by Pétain.