By the end of 1942, Ultra had become the richest and most reliable of the many different sources of intelligence, however, the relation between their individual contributions requires analysis. Ralph Bennett's study aims to present a succinct survey and analysis of the military intelligence available to Britain and her allies during World War II. When war began, Britain was as ill-prepared in intelligence as in armaments. Civilian scientists had discovered the principle of radar in the mid-1930s, but everything else had to be learned from scratch in the heat of emergency. In 1939, all three armed services lacked staff trained to appraise and distribute intelligence. Because it had so often been unreliable in the past, field commanders were reluctant to accept it and had to learn new ways. First signs of improvement came in mid-1941, when Ultra targeted naval vessels and bomber aircraft onto so many of Rommel's supply ships that the Afrika Korps almost withered on the vine. From then on intelligence played an increasingly indispensable part in final victory. Ultra won the Battle of the Atlantic, driving the U-boats back to coastal waters by June 1943. A lower grade code gave Montgomery the vital first news of the whereabouts of the German tanks as he planned the breakthrough at Alamein. Only "Bomber" Harris refused to give intelligence the credit it deserved, for reasons investigated in "Behind the Battle".