began to field self-propelled anti-tank weapons, first the M3 Gun Motor Carriage, and then the M10 tank destroyer. As a result of the Army's inability to resolve the questions of equipment and doctrine, it continued to struggle with efforts to develop viable anti-tank doctrine, and efforts to employ anti-tank guns or tank destroyers as part of combined arms team often proved ineffective. The ongoing debate within the Army about which type of anti-tank or tank destroyer weapon to use, and what design specifications ought to be included, also hampered the AGF's abilities to field the weapons and provide adequate training. As a result, during their initial employment and use, AGF observers in combat theaters identified several issues, including some as basic as training ammunition for the M3 Gun Motor Carriage being used unknowingly during combat, which obviously limited its effectiveness. Over time, commanders in combat resorted to field expedient measures to solve the problems, including learning to employ anti-tank guns and later tank destroyers in mutually supporting positions, and integrating them with infantry and armor units to maximize their effectiveness as part of a combined arms effort. The AGF in turn incorporated these lessons learned into mobilization training, so that over time soldiers and units deploying for combat were making use of the most up to date doctrine and tactics. Tanks At the start of World War II, the United States fielded the M3 Lee and M4 Sherman as its primary medium tanks. After-action reports from the North African Campaign and other engagements convinced commanders including Jacob L. Devers that the U.S. needed to deploy a heavier tank with more firepower in order to counter German Tiger I and Panther tanks. During his assignment as Chief of Armor earlier in the war, Devers had rejected the M6 heavy tank for being under-powered and under-gunned for its weight and size. As a result, the Army's Ordnance Department had overseen the creation of the T20 Medium Tank. In 1942, Devers advocated the immediate shipment of 250 T20s to the European Theater. McNair opposed this request, still convinced that smaller but heavily armed self-propelled tank destroyers could be employed faster and more effectively, especially when considering factors such as available space on cargo ships transporting weapons and equipment to Europe. The Army attempted development of an improved version of the T20, the T23, but design flaws kept it from being moved into production.