Following the surrender of the Leige Forts by the Belgian Army, the German Army continued its push towards Paris under the Schlieffen Plan. The remainder of the Belgian Army had begun to retreat to meet the newly arrived British Expeditionary Force, which was advancing to halt the German drive. The French were being pushed back and slaughtered on the southern end of the front so they were unable to assist the Belgians so the weight of the German advance fell on the small British army.
By August 22, the II Corp of the British Army, under Gen. Horace Smith-Dorrien, had reached the town of Mons and had established defensive positions.
At 6 A.M., on the 23rd, the advanced guard of the German First Army arrived on the outskirts of Mons. B company of the 4th Middlesex Regiment opened fire on the advancing German cavalry and forced them to fall back to the main body. The battle was under way.
Around 9 A.M, the German Infantry had arrived at Mons and were preparing their assault on the north end of the town. 8 German battalions, supported by artillery fire, attacked the 4th Middlesex and 4th Royal Fusiliers positions. On the first assault, the Germans attacked in close formation attack waves and were relentlessly shot down by the British infantry of the two regiments, which at the time was notably made up of the best rifle marksmen in the world. At one point in the battle, the German field officers reported to First army commander Gen. von Kluck that all the British were using machine guns because the British were shooting so rapidly and inflicting so many casualties on the advancing Germans.
Soon the engagement grew more desperate for the British as the rest of the First Army arrived on the scene. Artillery fire had pushed the Middlesex and Royal Fusiliers from their positions and the II corp was beginning to fall back under the weight of the German advance. Still, the British put up stubborn resistance.
By 2 P.M., the British soon realized that they were hopelessly outnumbered by the Germans and that it would only be a matter of time before they were overwhelmed. also, the British had been informed of the French Army’s retreat from their positions to the immediate east, which left the British flank exposed to German attack. The Germans had already begun to clear the town of the British forces there. Gen.Smith-Dorrien pulled his troops back to the south. When word reached him that the Belgian army was falling back, the British were forced to follow suit. The retreat from Mons had begun.
On the 25th, the British fell back to Le Cateau and set up defensive positions. Haig’s corps, which still had not seen battle, pulled in to support the battle weiry troops under Smith-Dorrien. By early morning the next day, the Germans had come up on the British and were heavily attacking the lines. The situation for the British became critical. The right flank of the line began to break and soon after the left. Only the arrival of French cavalry kept the line together. That night the British once again withdrew and moved to St. Quentin. There had been 7800 casualties out of 40,000 from the action at Le Cateau. Several of the British regiments had vanished from the rolls and the II Corps was in a seriously depleated condition.
The Belgian army then counterattacked the Germans to slow them down so that the British could reform and rest briefly. The Belgians and their French reserves managed to push the German 1st Army back far enough to temporarily stall the advance. Then the B.E.F. and the Belgians pulled out and retreated successfully. The stunned Germans recovered and changed the course of their advance and began to push south towards the Marne River and Paris.