12th Brigade formed part of 4th Division of the British Expeditionary Force (BEF), ready to go overseas in the event of hostilities. When the European War broke out in August 1914, the 4th Division proceeded to France as soon as its coastal defence duties could be handed over. The division arrived at the front on 26 August, hungry, wet and weary after a night march, and was rushed into action at the Battle of Le Cateau. Wilson's 12th Bde was taken by surprise and suffered heavy casualties, but rallied and held the extreme left of the British line until the BEF was able to retreat. On 9 September the commander of 4th Division was disabled by an accident and Wilson took over as acting GOC, leading it at the crossing of the Aisne on 13–14 September.