Editing M4 Sherman (section)

===The tank destroyer doctrine===
General [[Lesley J. McNair]] was head of the Army Ground Forces from 1942 to 1944. McNair, a former artilleryman, advocated for the role of the [[tank destroyer]] (TD) within the U.S. Army. In McNair's opinion, tanks were to exploit breakthroughs and support infantry, while masses of attacking hostile tanks were to be engaged by tank destroyer units, which were composed of a mix of self-propelled and towed anti-tank guns. Self-propelled tank destroyers, called "gun motor carriages" (as were any U.S. Army self-propelled armored vehicles mounting an artillery piece of heavy caliber), were similar to tanks but were lightly armored with open-topped turrets. The tank destroyers were supposed to be faster and carry a more powerful anti-tank gun than tanks (although in reality tanks often received more powerful guns before tank destroyers did) and armor was sacrificed for speed.{{sfn|Zaloga|2008|loc="McNair's Folly"|pp=72–77}} Armored Force and Tank Destroyer Force doctrine were developed separately, and it was not against Armored Force doctrine for friendly tanks to engage hostile tanks that appeared while attacking or defending;{{sfn|Moran (6 June 2015) ''Myths of American Armor''}} tank destroyers were to engage numbers of enemy tanks that broke through friendly lines.

McNair approved the 76&nbsp;mm upgrade to the M4 Sherman and production of the 90&nbsp;mm gun-armed [[M36 tank destroyer]], but he at first staunchly opposed mass production of the [[T20 medium tank]] series and its descendants, the T25 and T26 (which would eventually become the [[M26 Pershing]]) during the crucial period of 1943 because they did not meet the two criteria of the Army Ground Forces for accepting new equipment; they were not "battle worthy," and he saw no "battle need" for them. In fall 1943, Lieutenant General Devers, commander of U.S. forces in the [[European Theater of Operations]] (ETO), asked for 250 T26 tanks for use in the [[Operation Overlord|invasion of France]]; McNair refused, citing the fact that he believed the M4 was adequate. Devers appealed all the way to the War Department, and Major General [[Russell L. Maxwell]], the Assistant Chief of Staff G-4 of the War Department General Staff, ordered the 250 tanks built in December 1943. McNair finally relented in his opposition, but still opposed mass production; his Army Ground Forces even asked for the tanks to be "down-gunned" from 90&nbsp;mm to 75 or 76&nbsp;mm in April 1944, believing the 76&nbsp;mm gun was capable of performing satisfactorily. General [[George C. Marshall]] then summarily ordered the tanks to be provided to the ETO as soon as possible. Soon after the [[Normandy invasion]] in June 1944, General [[Dwight D. Eisenhower]] urgently requested heavy tanks, but McNair's continued opposition to mass production due to persistent serious mechanical problems with the vehicles delayed their procurement. That same month, the War Department reversed course and completely overruled the Army Ground Forces when making their tank production plan for 1945. 7,800 tanks were to be built, of which 2,060 were to be T26s armed with 90&nbsp;mm guns, 2,728 were to be T26s armed with 105&nbsp;mm howitzers and 3,000 were to be M4A3 Sherman tanks armed with 105&nbsp;mm howitzers. As a part of the plan, the British requested 750 90&nbsp;mm-armed T26s and 200 105&nbsp;mm-armed T26s.{{sfn |Zaloga|2010|pp=19–20}} General McNair was killed in a [[Operation Cobra|botched air support mission]] in July 1944, and the path to production for the T26 tank became somewhat clearer. General Marshall intervened again and the tanks were eventually brought into full production. However, only a few T26 tanks (by then designated M26) saw combat beginning in February 1945, too late to have any effect on the battlefield.{{sfn|Zaloga|2008|pp=120–125, 287}}