What happened to the Matildas, Crusaders, M3 Lees, BT-7s after they became obsolete at or around 1943? I know that some were still used in the China, India, Burma theatre or at the Pacific theatre and some were converted to SPAA or SPGs but that doesn't account for all of them. Where did they go?
I can answer this question from the Red Army perspective.
Old tanks generally weren't taken out of service entirely just because they were old. For example, the Red Army still had about 10,000 T-26 tanks on hand in 1941, including over 1200 two-turreted T-26 tanks and chemical tanks on the same chassis that were pushing a decade of service by that point. These tanks engaged in the fighting in the summer of 1941 just the same, since it's not like the Red Army had any other tanks to replace them with. Despite heavy losses, there were still quite a few of these tanks in service by the end of the year. While you were less likely to encounter a T-26 tank than a T-60 or T-34 in battles around Moscow, secondary directions like the southern part of the front and especially the Far East were still teeming with old tanks. I'll come back to the Far East in a little bit.
Compared to the dynamic nature of the central (Battle of Moscow, Rzhev, etc) and southern (Stalingrad, Battle of the Caucasus) directions, the northern direction remained fairly static since the terrain was very favourable for the defender. Until the blockade was lifted in 1944, the defenders of Leningrad had to make do with anything that had tracks and a gun. Even very old T-26 and BT-2 tanks remained in service. There were some local conversions such as the SU-26 (76 mm SPG on the T-26 chassis) but many tanks were also used as is. Even after the blockade was broken, repair factories continued to restore old tanks. They most likely never saw combat again, but there was no official declaration that these tanks were obsolete, and so they were still repaired as long as there were parts.
Other fronts continued to use old tanks too. For example, the USSR continued to receive Matildas from the UK until 1943, and the latest I'm aware of "new" Matildas being issued was 1944. This did create rather comical scenarios where brand new T-34-85 tanks were going into battle alongside obsolete pre-war designs. Unfortunately, the choice often lay between an old tank and no tank at all, and a Matilda could fight infantry just as well in 1944 as it could in 1940.
Now, back to the Far East. Soviet forces gunning towards Berlin had a way to lose tanks and thus order replacements, but Far Eastern forces didn't lose tanks and didn't gain them either. Far Eastern units had a wide variety of downright museum quality vehicles like T-27 tankettes and T-37/38 amphibious tanks. Work was done to upgrade some of them, for instance T-38 tanks received 20 mm autocannons and additional armour to make them useful against at least light Japanese tanks in battle and BT-7 tanks received additional armour.
Even as T-34-85s and other modern tanks poured into Far East units to prepare them for a war against Japan, the light tanks were retained as a reserve. A small quantity of BT-7 and T-26 even set out with the invasion force into Manchuria, although without much effect. The only mention I've seen in reports was that a lot of them unsurprisingly broke down very quickly and had to be towed back to Soviet bases. Even in August of 1945 these tanks that were over a decade old weren't simply abandoned.
After the war there was still a fairly large number of these obsolete tanks remained on hand. Since there were no more spare parts coming for either the Soviet or foreign tanks, permission was given to cannibalize broken ones to keep the others running. These tanks were used for training or as prime movers at army bases. Despite the army's insistence to keep the tanks in the army, a lot were handed off to the civilian sector. No particular effort was put in to rationalizing the existing spare part stocks. In fact, a decision was made to sell 4629 train cars full of spare parts for obsolete Soviet and Western tanks at scrap value to the agricultural sector or just scrapped directly.
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