Both sides in the Ukraine War have employed 1950s-era Soviet T-55 tanks. Russia brought in several hundred T-55s from stockpiles of older T-55 and T-62 tanks that are maintained by Russia because this is what they have always done and having some available for use in Ukraine proved useful.
The T-54 was the first modern Russian tank and entered service in 1947 as the T-44. This was actually a major upgrade of the World War II era T-34/85. The original T-34 that the Germans encountered in 1941 had a 76mm gun. This was later replaced with a more powerful 85mm weapon.
The T-34 was an unpleasant surprise for the Germans in 1941. It had excellent speed, good armor and was soon upgraded with an 85mm gun. Similarly, there was an upgrade (to include protection from nuclear radiation) to the T-54, as the T-55 which entered service in 1959. About 100,000 T-54/55 tanks were produced between 1967 and 1979. Thousands, mainly late model T-55s, are still in use in active (kept ready to use) storage.
In Ukraine the T-55s were used by both sides to support infantry attacks or as defensive pillboxes. This was accomplished by driving the T-55 into an open bunker so that only the turret was visible. These were excellent defensive weapons when facing an attack by enemy armored vehicles, which were fully visible while all the attackers could see was the T-55 turret. The bunkered T-55s were still vulnerable to artillery fire and top-attack anti-tank missiles.
In Ukraine there was a third use for T-55s, but only by the Russians. The crew was removed and the interior of the tank was filled with about five tons of explosives and a remotely controlled detonator. The driver controls were also converted to be operated remotely. These bomb tanks were carried by tank transporters to the front lines and unloaded for use against enemy positions. Several of these bomb tanks have been used, with spectacular effect. When the Ukrainians have suitable anti-tank weapons, they can hit the T-55 bomb tank before it gets close enough to detonate and cause the Ukrainians casualties.
The Ukrainians also have some of these old T-55s in service. But about a year ago NATO member Slovenia sent Ukraine 28 M55S tanks, which appear quite modern and they are. The M55S is a heavily modified T-55 tank. In 1999 Slovenia completed the conversion of 30 T55 tanks into the much superior M55S. The original 100mm cannon has been replaced with a western 105mm rifled gun. A new Israeli fire control system and explosive-reactive armor were added. The suspension was upgraded and new tracks installed. A modern 600MP diesel engine replaced the original Soviet 580HP diesel. The M55S is similar to the Israeli T55 upgrade that produces the Ti67. The original T55 weighed 36 tons and the M55S upgrade created a 38-ton tank.
Ukraine found the M55S useful because it was a modern tank, although not as heavy (50 tons), well-armed (120/125mm gun) or mobile (much more powerful engine) as current Russian (early T-72B3) or Western (M1/Leopard 2) tanks. The M55S did not have an auto-loader and was not as vulnerable to destruction as Russian tanks introduced since the late 1960s. Without the autoloader, the T-55 had a four-man crew, which made it easier to maintain and keep operational. The auto-loader vulnerability was not realized until the Ukraine War because Russian tanks had never faced so many top-attack ATGMs (Anti-tank guided missiles) or so many modern tanks with 120-125mm guns. The Russians only realized this when over half the modern (auto-loader) Russian tanks used during the first few months of the Ukraine War were destroyed or abandoned because of top-attack missiles. In effect, the auto-loader tanks were eight times more likely to be destroyed in combat than a modern Western tank. This means that the M55S is nearly as effective in combat as comparable 1960s era third-generation Western tank like the Leopard 1, M-60 or Chieftain).