News that German forces were attacking towards the Meuse bridges south of Namur, led Joffre to expect a German attack from Mézières to Givet, further north, intended to envelop the French northern flank and another force to try to cross the Meuse from Montmédy to Sedan. On 12 August, Joffre allowed Lanrezac to move the I Corps west to Dinant on the Meuse and on 15 August, Joffre ordered the bulk of the Fifth Army to move north-west behind the Sambre. No large German force was expected to cross to the north of the Meuse, which made the French general staff certain that the German centre was weaker than expected. On 18 August, Joffre directed the Third, Fourth and Fifth armies, together with the Belgians and British, to attack the German armies around Thionville and Luxembourg, where corps were thought to have assembled. The Third and Fourth armies were to defeat German forces between Thionville and Bastogne, as they attacked westwards towards Montmédy and Sedan. The Fifth Army was to intercept German forces advancing towards Givet and then the Fourth Army was to swing north and attack the southern flank of the German armies. The Third and Fourth armies would defeat decisively the main German armies in the west and for this, two more corps were added to the four in the Fourth Army, taken from the flanking armies.