The German plan of campaign was simple in its essence, although involving great numbers of men and an inconceivable mass of material. It was to strike the main blow along the Oise on the front between St. Quentin and La Fere, while a subsidiary attack was to be simultaneously delivered on the northern side of the Cambrai salient between Cambrai and Arras. This subsidiary attack was designed to break the salient and destroy the danger of a flank attack against the movement to the south. In the main attack, delivered with 15,000 men to the mile of front, it was intended to break the connection between the British and the French along the Oise, push a great wedge through at the point of rupture, and then roll the British line back to the north, leaving the French to be taken care of later. Failing in this (and Germany had taken into account the possibility of failure), the British were to be forced back through Amiens to the sea, and the split in the armies accomplished by interposing between the parts a section of the seacoast. This operation would automatically flank the positions held by the British at Arras, force the British to fall back from Vimy Ridge, and from Lens toward St. Pol, and, as they retreated, to uncover the Ypres salient and the positions held in the high ground to the east and south of Ypres--that is, the Messines and the Passchendaele ridges. [Sidenote: The Germans use eighty divisions the first day.] [Sidenote: The Allies retreat.] After a brief but very intense bombardment the German infantry went forward on March 21, 1918. They were favored by a heavy mist which concealed their movements until they were within fifty yards of the British trenches, between La Fere and St. Quentin. By sheer weight of numbers these trenches were overrun and the German infantry poured through the gap. The line to the north was at once affected by the break in the southern line, and taken in flank, was also forced to fall back. But a few hours after the attack was launched, the entire fifty miles of line north of La Fere was ablaze and the British were in retreat. In this attack the Germans threw in on the first day 80 divisions--about one million men--nearly 20,000 men to the mile--a heavier concentration of men than had ever been used in an attack since the war began. Against this number the British, in the opening attack could oppose only 5,000 men to the mile. It is not surprising in view of this disparity in numbers that the British were completely overwhelmed. In spite of the rapidity of the initial German advance and the strength of the German attack, the hoped-for rupture of the Allied line at the Oise did not occur. The British and French, though retreating steadily, kept in close touch and preserved intact the continuity of their line. [Sidenote: The French extend their left to keep in touch with the British.]