Amid clouds of gas, smoke and fog, the British in the advanced positions were surrounded and overwhelmed, often before they had realized what was happening. Nearly all their machine-guns, posted to sweep the first zone, were put out of action. The First Day (March 21). The first day of the attack, General Byng's Army from Fontaine-les-Croisilles to Demicourt, withstood the shock steadily, the Germans penetrating the first lines only. In the centre, before St. Quentin, and to the south, in front of Moy and La Fère, General Gough's Army, overwhelmed by numbers, and notwithstanding the courage of the men, was broken early in the attack. Opposite Le Catelet, the enemy storm divisions advanced 6 to 8 kilometres, penetrating at noon the second-line positions along the Epéhy-Le Verguier line. Further south, in front of Moy, they reached Essigny-Fargnières. General Gough withdrew his right behind the water-line of the Crozat and Somme Canals. [Illustration: _The Disruption of the Front. March 21-22._] The Second Day--March 22. Tergnier fell, and the water-line was turned from the right. Still favoured by the fog, the Germans crossed the Crozat Canal. Fresh divisions harassed the British without respite, the losses, both in men and material, being very heavy. Their reserves, greatly outnumbered, were quickly submerged, and the third positions were lost after a desperate but ineffectual resistance. In spite of its stubborn resistance, the 3rd Army (Byng) was forced to fall back, pivoting on its left, to line up with the retreating 5th Army (Gough). The enemy advance developed rapidly. Within forty-eight hours, over 60 German divisions (750,000 men) had been thrown into the battle, which now raged in the open. THE INTERVENTION OF THE FRENCH. [Illustration: _Arrival of the first French Divisions. (March 22)._ _Humbert's Army barring the road to Paris. (March 24)._ _Debeney's Army holding the enemy on the west. (March 28)._ PHASES OF THE FRENCH INTERVENTION.] The crushing of the right and centre of the British 5th Army opened a large breach north of the Oise, through which, as early as March 21, the Germans streamed south and west. The situation was critical, as the enemy hordes, having broken through the fortified zone, threatened to submerge all before them. Prompt intervention was imperative, in order to retard the enemy at all cost. [Illustration: GENERAL PELLÉ REVIEWING THE TROOPS OF THE 5TH CORPS IN 1917.] As early as the evening of the 21st, General Pétain made dispositions to support the British right. The 9th and 10th Div. (5th Corps) and the 1st Div. of unmounted Cuirassiers (Pellé), in reserve near Compiègne, received orders to hold themselves in readiness. At the same time, the staff of Gen. Fayolle's Army Group, and that of Gen. Humbert's Army, prepared to take over the direction of the operations.