A French offensive in the
Rhine river valley area (Saar Offensive) started on 7 September, four days after France declared war on Germany. Since the
Wehrmacht was occupied in the attack on Poland, the French soldiers enjoyed a decisive numerical advantage along their border with Germany. However, the French took no meaningful action to assist the Poles. Eleven French divisions advanced along a 32 km (20 mi) line near
Saarbrücken against weak German opposition. The attack did not result in the diversion of any German troops. The all-out assault was to have been carried out by roughly 40 divisions, including one
armored, three mechanised divisions, 78
artillery regiments and 40 tank
battalions. The French Army had advanced to a depth of 8 km (5.0 mi) and captured about 20 villages evacuated by the German army, without any resistance. However, the half-hearted offensive was halted after France seized the Warndt Forest, 3 sq mi (7.8 km2) of heavily-mined German territory.
On 12 September, the
Anglo French Supreme War Council gathered for the first time at Abbeville. It was decided that all offensive actions were to be halted immediately as the French opted to fight a defensive war, forcing the Germans to come to them. By then, the French divisions had advanced approximately 8 km (5.0 mi) into Germany on a 24 km (15 mi)-long strip of the frontier in the Saarland area. General
Maurice Gamelin, ordered his troops to stop no closer than 1 km (0.62 mi) from the German positions along the Siegfried Line. Poland was not notified of this decision. Instead, Gamelin informed
Marshal Edward Rydz-Śmigły that 1/2 of his divisions were in contact with the enemy, and that French advances had forced the
Wehrmacht to withdraw at least six divisions from Poland. The following day, the commander of the French Military Mission to Poland, General
Louis Faury, informed the Polish
Chief of Staff — General
Wacław Stachiewicz — that the major offensive on the western front planned for 17–20 September had to be postponed. At the same time, French divisions were ordered to withdraw to their barracks along the Maginot Line.