When the German offensive began Brooke, aided by Neil Ritchie, his Brigadier General Staff (BGS), distinguished himself in the handling of the British forces in the retreat to Dunkirk. In late May 1940 II Corps held off the major German attack on the Ypres-Comines Canal but then found its left flank exposed by the capitulation of the Belgian army. Brooke swiftly ordered Montgomery's 3rd Division to switch from the Corps' right flank to cover the gap. This was accomplished in a complicated night-time manoeuvre. Pushing more troops north to counter the threat to the embarking troops at Dunkirk from German units advancing along the coast, II Corps retreated to their appointed places on the east or south-east of the shrinking perimeter of Dunkirk.