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Battle of France

Montgomery's training paid off when the Germans began their invasion of the Low Countries on 10 May 1940 and the 3rd Division advanced to its planned position, near the Belgian city of Louvain. Soon after arrival, the division was fired on by members of the Belgian 10th Infantry Division who mistook them for German paratroopers; Montgomery resolved the incident by approaching them and offering to place himself under Belgian command, although Montgomery himself took control when the Germans arrived. During this time he began to develop a particular habit, which he would keep throughout the war, of going to bed at 21:30 every night without fail and giving only a single order—that he was not to be disturbed—which was only very rarely disobeyed.

The 3rd Division saw comparatively little action but, owing to the strict training methods of Montgomery, the division always managed to be in the right place at the right time, especially so during the retreat into France. By 27 May, when the Belgian Army on the left flank of the BEF began to disintegrate, the 3rd Division achieved something very difficult, the movement at night from the right to the left of another division and only 2,000 yards behind it. This was performed with great professionalism and occurred without any incidents and thereby filled a very vulnerable gap in the BEF's defensive line. On 29/30 May, Montgomery temporarily took over from Brooke, who received orders to return to the United Kingdom, as GOC of II Corps for the final stages of the Dunkirk evacuation.

The 3rd Division, temporarily commanded by Kenneth Anderson in Montgomery's absence, returned to Britain intact with minimal casualties. Operation Dynamo—codename for the Dunkirk evacuation—saw 330,000 Allied military personnel, including most of the BEF, to Britain, although the BEF was forced to leave behind a significant amount of equipment.