Mangin's army was ordered to strike with all its force. It was equipped with new and speedier 'whippet' tanks, and its immediate onset was masked by the accident of a July thunderstorm on the eve of the fighting. On the morning of 18th July it went forward with nothing but a barrage, but with the effect of a thunderbolt, and its average advance on that day was 5 miles, with Fontenoy and the plateau of Pernay on the Aisne firmly secured. Degoutte's army went forward for 2 miles over difficult country. A blow of immense significance had been struck along the whole length of the salient's vulnerable side. Next day Mangin's movement continued; he tightened his hold on the Aisne and swung his right wing farther along the Ourcq so as to bring the whole line of the one good north-and-south road in the salient under the fire of his guns, and thus to hamper the German movements terribly. That alone would have forced the enemy to begin a retreat from the Marne, while it might yet be possible. Meanwhile Degoutte was advancing also, and was forcing the Germans away from the neighbourhood of Château-Thierry. The German commander, von Böhn, was not slow to recognize the implications of the situation into which Foch had forced him, and gave orders to recross the Marne. He was in time, but his retreating troops were roughly handled at the crossings, and despite all his attempts to hold up the attack on his western flank by counter-attack, the pressure of Mangin and Degoutte, added to that of de Mitry's army, which was now following him back over the Marne, became every hour more dangerous. By 20th July not only was the Marne itself in process of being abandoned by von Böhn but Château-Thierry had fallen. Degoutte's army was 3 miles north of it. De Mitry's divisions had secured ample crossings for future movement and Berthelot's mixed forces of French, British, and Italians had begun a disconcerting attack on the eastern side of the salient. In three days the Germans lost 20,000 prisoners, and, what was more significant, 450 guns. The first _riposte_ of the French Generalissimo had been delivered. It was to be followed in unending succession by others.
The German Commander-in-Chief had to gain time. It was no easy thing to withdraw his 600,000 men crowded between the Aisne and the Marne, but he was obliged to support them lest the salient should collapse too suddenly. He aimed a counter-stroke at Gouraud's army east of Rheims, but the blow spent itself in the air, and Foch replied by setting in motion the army of General Debeney, where it stood opposite to that of General von Hutier, between Montdidier and Noyon. But he had not yet gathered the full fruits of the strength of his positional assault between the Aisne and the Marne, and did not in the least allow pressure here to relax. Mangin seized Oulchy-le-Château on the Ourcq (25th July); and on 26th July Gouraud, on the other side of Rheims, recovered the ground he had ceded under the German pressure. By Sunday (28th July) the Allied attack in the Aisne-Marne salient had swept convergingly on to the line of the Ourcq, and with that achievement the most important episode in the opening of Foch's campaign was consummated. Soissons fell to Mangin on 1st and 2nd Aug., a signal that the work was done, and Foch was now free to prosecute his larger plan of delivering successive blows at points where they would disperse and use up the German reinforcements most effectively.