The fortress of Maubeuge stood on the River Sambre, just inside France. In August 1914 it stood directly in the line of the great German sweep through Belgium that was at the heart of the Schlieffen Plan. Initially the French did not believe that any significant German forces were north of the Sambre, but when they finally realised what was going on, Joffre ordered the French Fifth army north to defend the line of the river. At the same time the BEF was approaching Mons, due north of Maubeuge, in preparation for a planned Allied advance further into Belgium.
The French move came too late to prevent the Germans gaining a foothold on the south bank of the Sambre on 21 August (battle of the Sambre). On 23 August, as the British were fighting at Mons, the Germans began to cross the Meuse, east of the French position, threatening to cut them off. General Lanrezac was forced to order a retreat. The developing German threat also forced the BEF to retreat, despite holding the Germans at bay during the day at Mons.
The British and French retreat left the fortress at Maubeuge isolated. Rather than abandon the place, on 25 August General Fournier was ordered to defend the fortress, in the hope that the defence might delay the Germans. Instead, the Germans detached the VII Reserve corps to besiege Maubeuge, while the rest of the army continued to advance into France.
Maubeuge was defended by fourteen forts, a garrison of 30,000 French soldiers (mostly territorials), and 10,000 Allied stragglers. The German VII Reserve corps contained two divisions, perhaps 34,000 men in total, supported by the efficient German heavy artillery.
On 29 August the Germans began a systematic bombardment of the forts around Maubeuge. On 5 September, after a week long bombardment, four of the fourteen forts were stormed by German infantry, creating a gap in the defences. On 7 September the garrison surrendered. The Germans took 40,000 prisoners and captured 377 guns.
On the same day that VII Reserve corps stormed the forts at Maubeuge, the first battle of the Marne began (5-10 September). By the end of the siege, the German advance was in crisis. Two days later, on 9 September, the leading German armies were forced to retreat, to take up a new line on the Aisne. During the first battle of the Aisne (13-28 September) the Allies came close to breaking through a gap in the German lines, but were stopped by the arrival of the German Seventh Army. One of the units that made up this army was the VII Reserve corps, which marched forty miles in twenty four hours, and reached the gap just one hour before the Allies.