Lets say that Britain would have withdrawn from WW2 in 1940 - with the end of the european war the USA would have concentrated exclusively on the Pacific. In Mai/June 1941 Germany would have attacked the USSR - how would this have gone if the USSR wouldnt have gotten ANY western help??
Here some numbers: From Autumn 1941 to Summer 1945 the USSR got through Lend Lease:
14 795 planes , 7056 tanks , 8218 Anti aircraft guns, 1800 Self propelled guns
35 089 Radio Stations, 380 135 Field Telephones
3 000 000 Tons of fuel
131 633 Submachine guns , 345 735 Tons of explosives
90 Cargo vessels , 105 U boat hunters ,197 Torpedo boats
7784 Ship engines, 1981 Locomotives, 11 155 Freight cars
51 503 Jeeps, 375 883 Trucks , 35 170 Motorcycles
8071 Tractors , 3 786 000 Tyres
2 800 000 Tons of steel
15 417 000 pair of Army boots, 4 478 000 Tonns of food
In the same time period 7869 planes and 4699 tanks came from britain and canada.
The list is NOT complete!
Source: Mark Harrison Soviet Planning in Peace and War 1938-1945 P. 258
Because Germany had to fight esat and west simultaneously its forces were allways split - here some examples:
In November 41 some 2500 german planes were in the USSR, but 900 were fighting the western allies.
In November 42 some 2450 german planes were in the USSR, but allready
1700 were fighting the western allies.
In July 43 some 2500 german planes were in the USSR while nearly 1800 were fighting the western allies.
Source: John Ellis World War II Databook P. 232, 233,238
Without western help the USSR would have probably lost - or at best reached a stalemate - what do you think?