During the Battle of Britain, Campbell served as an Air Raid Precautions warden in London. Also during the Blitz, he met and befriended Anglo-Welsh poet and fellow alcoholic Dylan Thomas, with whom he once ate a vase of daffodils in celebration of St. David's Day. Although Campbell was over draft age and in bad physical shape, he finally managed to be accepted into the British Army. He was recruited into the Intelligence Corps because of his knowledge of foreign languages and began training as a private with the Royal Welsh Fusiliers on 1 April 1942. Having completed basic training, Campbell was transferred in July to the I.C. Depot near Winchester, where he was trained in motorcycles. In February 1943, he was promoted to sergeant, and in March he was posted to British East Africa.