Britain declared war on Germany on 3 September 1939. The 3rd Division was sent to Belgium as part of the British Expeditionary Force (BEF). Montgomery predicted a disaster similar to that in 1914, and so spent the Phoney War training his troops to retreat safely rather than offensive operations. During this time, Montgomery faced serious trouble from his superiors for his attitude regarding the sexual health of his soldiers. However, he was defended from dismissal by his superior Alan Brooke, commander of II Corps. 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 the River Dijle and then withdrew to Dunkirk with great professionalism, returning to Britain intact with minimal casualties. During Operation Dynamo — the evacuation of 330,000 BEF and French troops to Britain — Montgomery had assumed command of the II Corps after Brooke had taken acting command of the whole BEF.