Explanation: Early on, when Nazi Germany first turned on the Soviet Union, their ally of convenience against Poland, the Sovs were caught off-guard and performed poorly. Many units were in the process of re-equipping (a process that was, thus, interrupted by rapid German advances), command was in disarray, and the entire front was in chaos for months. Literal millions of Soviet soldiers were lost in these opening stages - also the grounds for many later myths about the Soviet military. The T-34, for example, was actually superior to most German tanks - but between the chaos of the opening invasion, and the fact that the Sovs had been unreasonably secretive about the tank to their own forces, it could not be used to its full capabilities.
However, by 1944, the Soviet army was in fighting shape, having stopped the Nazis through sheer desperation at Stalingrad, and then marshalled their resources (and understanding that the war was on their goddamn doorstep) to reform the Soviet military machine to fight the war that was happening, not the one they hoped to happen in a few years. Between that and US Lend-Lease, in which the US lent massive amounts of material aid to the Sovs, especially trucks, the Soviet army then consistently pushed back and outperformed the Nazi invaders, culminating in the end of the war with the red flag raised over Berlin.