After many months of relative inactivity (the "Phoney War" period), on 10 May 1940, Germany launched its assault on the Western Front. Barker was on leave in England and immediately returned to France. The division advanced to Brussels but following a strategic reverse in the campaign, was forced to retreat soon afterwards. On 27 May, Barker's brigade was briefly transferred to the 5th Division and fought to hold the Ypres-Comines canal against an assault by three German divisions. The day after, Barker and his brigade were ordered to retreat to Dunkirk. His brigade sustained heavy losses during the campaign, and was evacuated to England on 31 May. The historian Peter Young, then a platoon commander in Barker's brigade, was impressed by Barker during this period, and wrote he was "the coolest man you could wish to see under fire. He seemed to like it; indeed I feel sure that he did. He liked to observe his officers and men in times of stress, because it helped to weigh them up. After so many years I am not ashamed to confess that I always felt braver when he was present".