There are three main reasons why the Soviets were successfull against the Nazis;

1. They only had one front to fight and Stalin was deeply afraid that if the Japanese were to attack them the Soviet Union could not cope (remember the US had two fronts and two oceans to win control of),

2. The Soviet inhuman use of their own personnel blunted their flaws in tactics (massed human waves with few weapons against tanks, starvation of “lesser” groups, mass executions to instill fear, ect.)

3. American (Allied) aid, which totalled:

22,206 Aircraft
151,052 1.5 ton trucks
200,662 2.5 ton trucks
77,972 Willys Jeeps
15,000,000 pairs of Boots
380,135 Field phones
40,000 Radios
1,235,000 miles of Telephone cable
107,000,000 square yards of Cotton cloth
4,500,000 tons of Foodstuffs
49,000 tons of Leather
35, 170 Motorcycles
1,981 Locomotives
11,155 Rolling stock
12,755 Tanks
8,701 Tractors
361,903 Trucks

All of which was given when the Soviet industry had collasped and these were transported by non-Soviet ships across a the Alantic fulled with German U-boats.

If it wasn’t for the US industrial output and our determination to face the danger the Nazi’s posed, the Soviets wouldn’t not have survived, let alone have reached Berlin (which we allowed them to do).