On 10 April 1940, seven months after the outbreak of the Second World War, MacMillan was promoted to acting lieutenant-colonel and appointed as GSO1 in HQ 55th (West Lancashire) Motor Division, a first line Territorial Army (TA) formation. The division was commanded by Major-General Vivian Majendie, who was some eleven years MacMillan's senior, and was serving in Northern Command (soon moving to Eastern Command). The division was a motorised infantry formation composed of only two, rather than three, brigades, and was amongst several responsible for coastal defence and for engaging any possible enemy airborne landings in the event of a German invasion. In late June, after the British Expeditionary Force (BEF) was evacuated from Dunkirk and the fall of France, the division – composed initially of the 164th and 165th Infantry Brigades with supporting divisional troops – was reorganised as a standard infantry division with the addition of the 199th Infantry Brigade from the disbanded 66th Infantry Division.