Lesley McNair: Unsung Architect of the US Army | 2023 International Conference on WWII 

Nothing ?

You would have to be stupid to think individual replacement worked. All you are doing is feeding people into the meat grinder. The veterans survive, but not the new ones. You could just as well throw them overboard in the middle of the Atlantic. Some unit lost more than 100 % of their soldiers. You pull out the unit, refill and train it. The veterans are the backbone who helps the newbies survive. The individual replacement sort of camouflaged the fact that they ended up loosing more people and had pretty large number of deserters due to excessive time in battle. So all in all they probably ended up worse while it looked "good".

The M4 replaced the M-46 in Korea. The M-46 was not finished.

@tarjei99 That is woefully incorrect. The M-26 was not replaced by the M-4 in Korea. On the contrary the M-26 was used by several tank battalions going into Korea. By the Korean War the US Army was transitioning its Tank units to M-26 and the new M-46 Patton. By the end of 1950, 200 M46 Pattons had been fielded, forming about 15% of US tank strength in Korea; the balance of 1,326 tanks shipped to Korea during 1950 were 679 M4A3 Shermans (including the Easy 8 M4A3E8 variant), 309 M26 Pershings, and 138 M-24 Chaffee light tanks.  Subsequent shipments of M46 and M46A1 Pattons allowed all remaining M26 Pershings to be withdrawn during 1951, and most Sherman equipped units were replaced by the M-46A1. The reason for this is that very early in the Korean War there was there were only two tanks that could handle the T-34 Tank in combat. The M-26 and the Easy 8 version of the M4 Sherman with the 76mm high velocity gun. No other version of the Sherman could stand toe to toe with the T-34 and win. The Sherman DID NOT replaced the M-26 but was supplemented by several tank types. The reason was that the US Army had been drawn down in size after WWII. In the Far East most Army and Marine Units there were on occupation duty in Japan and had not even trained for Combat in years. The primary occupation Army in this period was the 8th Army (US). In October 1948 General Walton Walker was sent to Japan and ordered by General Douglas MacArthur to restore the combat Readiness of the 8th Army. Walker commenced one of the greatest post War rear area operations in history to rebuild and retrain the 8th and get it fully equipped. The 8th's tank units were woefully undermanned and under equipped as occupation duties in Japan didn't call for them. So Walker set about combing Pacific battlefields for every tank he could find which was primarily the M4 Easy 8 variant and the few M26s in theater. All these tanks were brought to Japan and put into Depot maintenance for renewal and brought up to combat ready status primarily by the Daewoo Company of Japan. By the start of the Korean War in June 1950, the 8th Army was primarily trained but had not yet fully completed its rearming, retraining and reequipping cycle. Additionally, the 8th was still performing occupation duties as well. So when Korea started Walker sent what ready units he had, specifically 4 Divisions led by the 24th Infantry Divisions Task Force Smith, a composit unit of Infantry, tank and Artillery units to hold the line till the rest of the Army could get there. Led by MG William F. Dean this unit was destroyed but held on long enough so Walker could establish his 4 Division Army in South Korea and mount a credible defense. So the tanks supplied to this Army was literally whatever was available. Additionally, the pre-Korean War Army was not up to combat standard as most of it was on occupation duty both in Europe and Japan. But once China was taken over by the Communists and Russia began to get aggressive in Europe and backed the North Korean Communists the US Military began to get ready for war and congress appropriated more money to get the Army back up to combat ready standards. Walker was George Pattons premier Tank Corps Commander in WWII and understood what needed to be done and was able to get the Army trained and equipped to do the job. And that included getting our tank forces fully equipped with whatever was available and get it into depot and made combat ready which included both M-4s, M-24, M-26 and even M-46s. By the end of 1951 all US Tank Forces in Korea had been issued M46A1s and what M4s were left were given to the South Korean Army. The M-24s were sent back to the US and given to National Guard units or other Allied Armies. Now for the big surprise. The M-26 became the M-46. After World War II, most U.S. Army armored units were equipped with a mix of M4 Sherman and M26 Pershing tanks. Designed initially as a heavy tank, the M26 Pershing tank was reclassified as a medium tank after the war. The M26 was a significant improvement over the M4 Sherman in firepower and protection. Its mobility, however, was deemed unsatisfactory for a medium tank, as it used the same engine as the much lighter M4A3 and was plagued with an unreliable transmission. Work began in January 1948 on replacing the original power plant of the M-26 with the Continental AV1790-3 engine and Allison CD-850-1 cross-drive transmission. This design was initially called the M26E2, but modifications continued to accumulate; eventually, the Bureau of Ordnance decided that the tank needed its own unique designation, the M46.The upgraded M26 received a new power plant and a main gun with a bore evacuator. Upon completion of the first model of the Detroit Tank Arsenal production line in November 1948, the M46 was christened after the late General George S. Patton Jr. By December the Army had ordered several hundred.In July 1950 Detroit Arsenal was producing Pershings and M46s at a rate of over a dozen a day. In August 1950 President Harry S. Truman authorized funding for increased M46 production as part an expansion of heavy tank development program. So the M-26 Pershing essentially was put in a product improvement program and was redesignated the M-46 Patton.

@johnpitchlynn9341 The M26 was replaced because it was underpowered. Korea has mountains. That does not preclude it being further developed into the M46.