[1] A series of encounter battles began between the German, French and Belgian armies on the German–French frontier and in southern Belgium on 4 August 1914.

The opposing armies met in thick fog and the French mistook the German troops for screening forces.

Further west, the Fifth Army had concentrated on the Sambre by 20 August, facing north on either side of Charleroi and east towards the Belgian fortress of Namur.

[2] By 20 August, the Fifth Army had begun to concentrate on a 40 km (25 mi) front along the Sambre, centred on Charleroi and extending east to the Belgian fortress of Namur.

On the left flank, the Sordet Cavalry Corps linked the Fifth Army to the British Expeditionary Force (BEF) at Mons.

The German 3rd Army crossed the Meuse and launched an attack against the French right flank, held by I Corps (General Louis Franchet d'Esperey).

A retreat was ordered towards evening by the 5th Division to conform and by nightfall II Corps had established a new defensive line, running through the villages of Montrœul, Boussu, Wasmes, Paturages and Frameries.

Abercrombie and his men then came under heavy fire from Germans concealed in the village and the order was given to retreat through the surrounding fields.

Despite the heavy German fire and the difficulty of communication in the close terrain, the retreat was carried out in an orderly fashion, although six officers and 280 men were reported as still missing on 29 August, including Abercrombie.

The pursuit by the 2nd Army was ordered to continue on 27 August through Landrecies and Trélon, with the X Reserve Corps advancing towards Wassigny.

[18] During the morning the 5th Cavalry Brigade moved to the west bank of the Oise about 2 mi (3.2 km) east of Cerizy (Moÿ-de-l'Aisne).

The British immediately pursued around the eastern flank and met mounted cavalry near Moy; the 12th Royal Lancers forced the Germans to dismount with rifle fire and stampeded their horses.

A squadron of mounted lancers got within 50 yd (46 m), charged and inflicted 70–80 casualties with swords and lances for a loss of five killed.

The Germans had expected to meet a weak infantry detachment and attacked with three dismounted squadrons, intending to charge with three more.

The Germans eventually managed to disengage and withdraw behind a hill north of the woods; during the evening the British retired to the south.

[20] At 9:00 a.m., General Otto von Garnier heard reports that Crépy and Béthisy were occupied and broke off the engagement, to rally east of Néry; the 4th Cavalry Division then moved south via Rocquemont to Rozières.

[25] The 6th Division of the III Corps, crossed the Aisne at Vic on 1 September and engaged the 3rd Cavalry Brigade at Taillefontaine, about 5 mi (8.0 km) north-west of Villers and drove it slowly back towards the village.

By the end of the day the 1st Army headquarters had abandoned hope of cornering large British forces south of Verberie, Crépy-en-Valois and Villers.

German infantry made slow progress through the Oise marshes amidst extensive artillery bombardments by both sides.

By early afternoon, aircraft reconnaissance reports showed that the French had begun to withdraw and Bülow ordered a pursuit by small infantry parties with field artillery, while the main force paused to rest, due to exhaustion and to concern that the fortress of La Fère obstructed a general advance and should be masked, while the 1st Army enveloped the French from the west and then attacked on 1 September.

[30] On 5 September the Sixth Army advanced eastwards from Paris against the German IV Reserve Corps, which had moved into the area that morning.

Overnight the IV Reserve Corps withdrew to a better position 10 km (6.2 mi) east and French air reconnaissance observed German forces moving north to face the Sixth Army.

The BEF advanced from 6–8 September, crossed the Petit Morin and captured bridges over the Marne and established a bridgehead 8 km (5.0 mi) deep.

By 10 September the German armies west of Verdun were retreating towards the Aisne and the Franco-British were following-up, collecting stragglers and equipment.

On 12 September, Joffre ordered an outflanking move to the west and an attack northwards by the Third Army to cut off the German retreat.

The pursuit was too slow; on 14 September the German armies had dug in north of the Aisne and the Allies met trench lines rather than rearguards.

The Germans had reached high ground on the north bank of the Aisne and begun to dig in, which limited the French advance from 15–16 September to a few local gains.

The 2nd and 9th Cavalry divisions were dispatched as reinforcements next day but before the retirement began, the French attack reached Carlepont and Noyon, before being contained on 18 September.

The main German effort remained on the western flank, which was revealed to the French by intercepted wireless messages.