It was intended that the division would remain in the United Kingdom to complete training and preparation, before being deployed to France within twelve months of the war breaking out.

The division spent little time training and its soldiers were dispersed and used to guard strategically important and vulnerable locations across North East England.

To boost morale, provide additional labour and guards for the rear echelon of the BEF, and score political points with the French Government and military, the division was sent to France in April 1940, leaving behind most of its administration and logistical units as well as heavy weapons and artillery.

The Chief of the Imperial General Staff, Edmund Ironside, secured a promise from the BEF that the division would not be used in action owing to it being untrained and incomplete.

When Germany invaded Belgium and the Netherlands, the BEF and French armies moved to meet the attack, leaving behind the 23rd Division to continue guarding airfields.

[4] On 29 March, the British Secretary of State for War Leslie Hore-Belisha announced plans to increase the part-time Territorial Army (TA) from 130,000 men to 340,000, doubling the number of divisions.

At that time 34,500 men, all aged 20, were conscripted into the regular army, initially to be trained for six months before being deployed to the forming second-line units.

[34][35] As 1939 turned into 1940, the division became caught up in an effort to address manpower shortages among the British Expeditionary Force's (BEF) rear-echelon units.

[37] The lack of such men had taxed the Royal Engineers (RE) and Auxiliary Military Pioneer Corps (AMPC) as well as impacting frontline units, which had to be diverted from training to help construct defensive positions along the Franco-Belgian border.

The intent was that by August their job would be completed and they could return to the United Kingdom to resume training before being redeployed to France as front-line soldiers.

[46] Historian Tim Lynch commented that the deployment also had a political dimension, allowing "British politicians to tell their French counterparts that Britain had supplied three more infantry divisions towards the promised nineteen by the end of the year".

He reluctantly caved to the political pressure to release the divisions, having been assured by General Sir John Gort (commander of the BEF) that the troops would not be used as frontline combat formations.

This initiated the Battle of Sedan and threatened to split the Allied armies in two, separating those in Belgium from the rest of the French military along the Franco-German border.

Demonstrating the division's lack of mobility, when the 10th Battalion, DLI was assigned to guard airfields near Abbeville (south of the River Somme) it required all of the divisional transport and immobilised all other units to accomplish this move.

[53][54] Once the Allied commanders had realised that the German crossing of the Meuse had turned into a major breakthrough, the BEF and French armies began a fighting withdrawal from Belgium back to France.

On 17 May, the lack of French reserves prompted Général Alphonse Joseph Georges, commander of all Allied forces in North-East France, to order the 23rd Division to be deployed to the new frontline along the Canal du Nord to face the German breakthrough.

[34][61][65] Additional French troops were supposed to be covering the division's southern flank, but due to the swift nature of the German advance they never arrived along the canal.

[62][65] The author Hugh Sebag-Montefiore described the state of the unfinished canal: "there was no water in it, and the deep ditch that was supposed to hold up the German armour would not have challenged a car in some places, let alone a tank".

[66] At this point, a composite battery of eleven field guns and two 4.5-inch (110 mm) howitzers were assigned to the division, coming from a Royal Artillery (RA) training camp near Arras.

The individual was briefly interrogated by Lieutenant-Colonel Hugh Swinburne and the battalion's French liaison officer before a summary execution was carried out by Sergeant Dick Chambers.

The 69th Brigade moved and took up position along the River Scarpe, northeast of Arras while the 8th Royal Northumberland Fusiliers (the division's motorcycle reconnaissance battalion) entered the town itself to reinforce the garrison.

[78] Ellis wrote that the "12th and 23rd Divisions ... had practically ceased to exist" as a result of the fighting that saw the "whole tract of country between the Scarpe and the Somme" fall into German hands, and "the British lines of communication ... finally cut; and the way to the Channel ports ... open".

But it may be perhaps accepted, with this important rider – at this time every single hour's delay was of incalculable service to the rest of the British Forces in France".

[79] David Fraser likewise wrote " ... these battalions, ill prepared, under-equipped and unsupported nevertheless on occasion held up the enemy for several hours".

[80] Under mortar fire and with German troops on the opposite bank of the Scarpe, the 69th Brigade withdrew towards Farbus and Vimy Ridge, north of Arras.

[81] Their stay at Vimy Ridge was short-lived due to the ongoing artillery fire, and the brigade moved closer to Arras taking up position at Roclincourt.

[82][86] Marley Force departed their camp, escorted by armoured cars and light tanks, but were soon turned around on the orders of Herbert, with no reason provided.

Due to ongoing aerial bombardments that blocked the roads and inflicted casualties, only the 6th Green Howards managed to arrive at the brigade's destination.

[93] From Bergues, the battalion was sent to Haeghe-Muelen, 8 miles (13 km) south east of Dunkirk, to bolster the garrison composed of Irish and Welsh Guards protecting the right flank of the main withdrawal corridor to the port.

Scavenging abandoned small arms, anti-tank rifles, and ammunition, the battalion held their position for a further 48 hours, suffering casualties and fending off German attacks.