The campaign against the Low Countries and France lasted less than six weeks. Germany attacked in the west on May 10, 1940. Initially, British and French commanders had believed that German forces would attack through central Belgium as they had in World War I, and rushed forces to the Franco-Belgian border to meet the German attack. The main German attack however, went through the Ardennes Forest in southeastern Belgium and northern Luxembourg. German tanks and infantry quickly broke through the French defensive lines and advanced to the coast.
Kluck’s change of direction meant the inevitable abandonment of the original wide sweep around the far (western) side of Paris. Now the flank of this wheeling German line would pass the near side of Paris and across the face of the Paris defenses into the valley of the Marne River . The premature inward wheel of Kluck’s 1st Army before Paris had been reached thus exposed the German extreme right wing to a flank attack and a possible counter-envelopment. On September 4 Moltke decided to abandon the original Schlieffen Plan and substituted a new one: the German 4th and 5th armies should drive southeastward from the Ardennes into French Lorraine west of Verdun and then converge with the southwestward advance of the 6th and 7th armies from Alsace against the Toul–Épinal line of fortifications, so as to envelop the whole French right wing; the 1st and 2nd armies, in the Marne valley, should stand guard, meanwhile, against any French countermove from the vicinity of Paris. But such an Allied countermove had already begun before the new German plan could be put into effect.