Battle of Montdidier and Lassigny

The battle of Montdidier and Lassigny was launched by General Mangin in order to widen the zone of rupture opened by the Allied victory at Amiens. While the British and Canadians drove into the German lines further north, French troops engaged in a local offensive to dislodge German forces solidly entrenched in the wooded Lassigny massif. The rapid success of the operation contributed to further weakening the German front.

After the success of the Second Battle of the Marne and the launch of the Hundred Days Offensive in early August, the Allies sought to maintain constant pressure on German forces. The operation on Montdidier and Lassigny was entrusted to Mangin, known for his brutal attacks and outflanking manoeuvres. The targeted zone was heavily defended, notably Mont de Lassigny, a strategic position blocking access to Picardy.

The offensive began with intense artillery preparation during the night of 9–10 August. On the morning of the 10th, several French divisions attacked on a narrow front, using infiltration by small groups and close artillery support. Renault FT tanks were engaged in cleared sectors, while Mangin's infantry, supported by aviation, advanced through forests and ravines. Despite German resistance on the heights, French forces succeeded in encircling the Lassigny massif and taking the eponymous hill on 12 August.

This victory, though local, had important effects: it deprived the Germans of a dominant position and secured the left flank of the Allied offensive toward the east. It contributed to the erosion of German tactical cohesion, already undermined since the Marne. It also confirmed the fully restored offensive capacity of the French army in 1918. The battle is one of the milestones of the Hundred Days Offensive that would lead to the progressive collapse of the German front.