However, mating the turbosupercharger with the Allison V-1710 proved to be far more problematic. As a result, designers of the fighter planes that utilized the V-1710 were invariably forced to choose between the poor high altitude performance of the V-1710 versus the increased problems brought on by addition of the turbosupercharger. The fates of all of the V-1710 powered fighters of World War II would thus hinge on that choice. The original XP-39 was built with a turbosupercharged V-1710. Numerous changes were made to the design of the production version (after a review by aerodynamicists at Langley Field), including a decision to drop the turbosupercharger. This decision came out of a combination of the severe teething problems encountered with the turbosupercharger mated to the V-1710, as well as the belief of the Army at that time that high altitude performance was not necessary in a fighter plane. The P-39 was thus stuck with poor high altitude performance and proved unsuitable for the air wa r in Western Europe, which was largely conducted at high altitudes. The P-39 was rejected by the British, used briefly by the U.S. in the early Pacific air war, and then was exiled by the USAAF to the Soviet Union under the Lend Lease program. The Soviets were still able to make good use of P-39s because of its excellent maneuverability and because the air war on the Eastern Front in Europe was largely short ranged, tactical, and conducted at lower altitudes. The P-40, which also had only the single stage, single speed supercharger V-1710, would have similar problems with high altitude performance.