Shortly after war began, Haddon, who on 3 September was promoted to the acting rank of captain, transferred to the 1st Battalion, Borders, then serving in England as part of Brigadier Noel Irwin's 6th Infantry Brigade, itself one of three brigades forming Major General Charles Loyd's 2nd Infantry Division, and was made Officer Commanding (OC) "B" Company. Along with the rest of the division, Haddon's battalion was sent to France in late September where it became part of the British Expeditionary Force (BEF). His rank of captain being made temporary on 3 December, in February 1940 he was made adjutant of his battalion, which in May was transferred to the 125th Brigade, part of the 42nd (East Lancashire) Infantry Division, a Territorial Army (TA) formation, which had only recently arrived in France. Subsequently, when the German Army invaded France three months later, Haddon was involved in the rearguard action towards Dunkirk in late May, from where the battalion, by now having suffered heavy losses and much reduced in men and equipment, were eventually evacuated from Dunkirk to England (see Dunkirk evacuation).