The P-40's lack of a two-speed supercharger made it inferior to Luftwaffe fighters such as the Messerschmitt Bf 109 or the Focke-Wulf Fw 190 in high-altitude combat and it was rarely used in operations in Northwest Europe. However, between 1941 and 1944, the P-40 played a critical role with Allied air forces in three major theaters: North Africa, the Southwest Pacific, and China. It also had a significant role in the Middle East, Southeast Asia, Eastern Europe, Alaska and Italy. The P-40's performance at high altitudes was not as important in those theaters, where it served as an air superiority fighter, bomber escort and fighter-bomber. Although it gained a postwar reputation as a mediocre design, suitable only for close air support, more recent research including scrutiny of the records of individual Allied squadrons indicates that this was not the case: the P-40 performed surprisingly well as an air superiority fighter, at times suffering severe losses, but also inflicting a very heavy toll on enemy aircraft. Based on war-time victory claims, over 200 Allied fighter pilots – from the UK, Australia, New Zealand, Canada, South Africa, the US and the Soviet Union – became aces flying the P-40. These included at least 20 double aces, mostly over North Africa, China, Burma and India, the South West Pacific and Eastern Europe. The P-40 offered the additional advantages of low cost and durability, which kept it in production as a ground-attack aircraft long after it was obsolescent as a fighter.