In this preliminary assault the Germans employed some ninety divisions or over 750,000 men on a front of about fifty miles. This was the heaviest concentration of men to the mile that had so far been used in campaigns on the western front. Against these forces the British had only about 5,000 men to the mile or less than a third of the number the Germans had in action. Having disposed of the Cambrai salient the Germans had opened the way for the real attack south which was made in the angle between the Oise Canal and the Somme. The assault was carried out with great masses of men in close formation, and the Allies' lines were overwhelmed in the first rush. But the British and French made an orderly if hurried retreat and their front remained unbroken. The Allies made the Germans pay heavily for every gain, fighting on steadily from point to point. The British during the retreat still clung to the southern bank of the Oise, but were finally forced across the river at Noyon which fell to the Germans. Meanwhile in the north the British were slowly falling back on the old battle field of the Somme. It was wisely decided by the British High Command to retire rather than bring forward the strong armies they held in reserve back of the lines. The use of these armies would eliminate the possibility of a great counterattack. Having reached the old battle field and after the Ancre valley was passed, the British ceased to retreat and established themselves strongly on the west bank of the river. South of the town of Albert the Germans made more important gains. They had reached and passed the Allies' old line as it existed before the fighting on the Somme. But in the last days of March they made little progress as their artillery had not kept pace with the forward rush and until the guns arrived no important advance could be made. The French had halted behind the Avre River, an excellent position, for there were wide marsh belts lining either bank of the stream. Before the French ceased to retreat the Germans had pushed their advance westward and encircled Montdidier, producing by doing this a salient in their lines with the town as the apex. One German flank extended eastward through Lassigny to the Oise and the other along the Avre and behind it. This salient marked almost the extreme limit of the German push. North of the Somme and south of Arras the German advance was checked, because the British held such strong positions on Vimy Ridge and the heights of Notre-Dame-de-Lorette. The Germans could not push on north of the river before they had reduced these strong and commanding positions. To clear the way they made a determined attack with 90,000 men on a small front of not more than five miles east of Arras. After an intense artillery fire that lasted all day the Germans repeatedly attacked, but met with repulse. They gained a few hundred yards at several points, but the British line remained practically intact.