It was raised on 2 September 1939, the day before war was declared, as part of the Territorial Army (TA) and served in the United Kingdom and later North-West Europe from June 1944 to May 1945.

To avoid war, the British Prime Minister Neville Chamberlain met with German Chancellor Adolf Hitler in September and brokered the Munich Agreement.

This resulted in 34,500 twenty-year-old militiamen being conscripted into the regular army, initially to be trained for six months before deployment to the forming second-line units.

Some TA divisions had made little progress by the time the Second World War began in September; others were able to complete this work within a matter of weeks.

[17][18][19] Due to the lack of official guidance, the newly formed formations were at liberty to choose numbers, styles, and titles.

Soldiers, aged 19, were reassigned to other formations; the Ministry of Labour allocated other men to essential industries; and medical standards weeded out those considered unfit.

On 30 September, after the requisition of civilian transport, the division moved to the Scottish Borders, south of Edinburgh, to start training.

[25] In October 1939, the Commander-in-Chief, Home Forces, General Walter Kirke, was tasked with drawing up a plan, codenamed Julius Caesar, to defend the United Kingdom from a potential German invasion.

[31] When the campaign ended in failure, the division was ordered to move south into England to make room for the returning troops.

[36][37] The division co-operated with the forming Local Defence Volunteers, laid landmines, and erected anti-invasion obstacles within its operating area.

[41][42] Due to the large number of men allocated to the infantry, in 1941, the British Army instituted reforms to build-up other arms and formations.

[49][50][51] The remainder of the 1943 and the opening of 1944 was used to conduct extensive training and divisional exercises, as the division had been assigned to partake in Operation Overlord, the invasion of German-occupied France.

In February 1944, General Bernard Montgomery, commander of the 21st Army Group, the main Allied formation in Operation Overlord, visited the division and addressed the men.

Operation Epsom intended for the corps to attack to the west of Caen, cross the Odon and Orne rivers, capture an area of high ground near Bretteville-sur-Laize and thereby encircle the city.

Despite delays caused by minefields and German holdouts in the forward area, the brigades captured most of their objectives before noon: La Gaule, Le Haut, and Cheux.

The rest of the brigade cleared Colleville, captured Tourville-sur-Odon, destroyed additional German tanks, and by the afternoon had seized the bridge across the Odon at Tourmauville.

The back and forth fighting, which spread to both sides of the Odon, continued through 29 June and resulted in the division fending off the attacks and was able to secure additional territory.

[61][62] The historian Lloyd Clark placed the division's defensive success on "careful positioning", taking advantage of terrain, as well as "excellent leadership and tactical prowess at the small unit level.

"[63] Due to heavy casualties suffered by the 2nd Battalion, Argyll and Sutherland Highlanders, they withdrew and the division ceded control of Gavrus on 30 June.

German resistance and counterattacks, heavy fighting, flanking fire, and a direct hit on one of the brigade's tactical headquarters caused delays and communication breakdowns.

In support, the British Army launched Operation Bluecoat that aimed to secure the American flank and reach the town of Vire.

It was intended to be captured by 09:55, but it took six-hours to achieve this due to mines, traffic jams, heavy fighting, and the methodical clearing of the village.

During this period, MacMillan was wounded by shell fire and was replaced by the 46th Brigade commander, Colin Muir Barber, who was made a major-general.

[76][77][17] On 6 August, the leading elements of the division reached Estry and the nearby Hill 208, and fought a back and forth battle for both locations over the following days.

This action was credited with diverting German resources away from Joe's Bridge, where XXX Corps began their assault from at the start of Operation Market Garden.

[83] Operation Market Garden intended to land the First Allied Airborne Army behind German lines to seize six bridges and other key areas, to facilitate an advance by XXX Corps through the Netherlands and across the Rhine and into Germany.

[84] Consequently, the 15th Division crossed the Wilhelmina Canal (Wilhelminakanaal), unopposed on 21 September, and advanced towards the village of Best, on the northwest outskirts of Eindhoven.

Prior to the 15th (Scottish) being committed to this, Montgomery assigned the division back to XII Corps that been tasked with attacking west from the corridor.

On 30 October, the division entered Asten, south of Helmond, in response a German counterattack launched to the east of Eindhoven.

The 15th (Scottish) was the only division of the British Army during the Second World War to be involved in three of the six major European river assault crossings; the Seine, the Rhine and the Elbe.