Just to put things in perspective:
Germany might have been able to deal with the Soviet Union alone, had they not involved Great Britain and the US, and mobilized their industry earlier than 1943. Most people don't realize that Germany did not go into full wartime scale, 24 hour production, until about the third quarter of 1942. If you look at the production figures for Germany and the Soviet Union between 1940 and 1945, you will see that in 1943 and 1944, the only two full years of production under full around the clock schedule, at least materially, Germany was able to keep pace with the Soviet Union. And even with the late start, kept up fairly well, overall*. And, this does not factor in any production disruption cause by allied bombing which would not have taken place if the U.S. and Britain were not involved.
People that think the Soviet Union could have dealt with Nazi Germany alone are not being totally realistic.
Germany
Rifle production (in thousands):
1940 - 1,352
1941 - 1,359
1942 - 1,370
1943 - 2,275
1944 - 2,856
1945 - 665
Total - 9,877
Machine gun production
1940 - 119
1941 - 325
1942 - 232
1943 - 234
1944 - 229
1945 - 78
Total - 1,156
Artillery (not incl mortars)
1940 - 6
1941 - 22
1942 - 41
1943 - 74
1944 - 148
1945 - 27
Total - 318
Tank and SPG production
1940 - 2.2
1941 - 3.8
1942 - 232
1943 - 6.2
1944 - 19.3
1945 - 4.4
Total - 45.6
Combat aircraft
1940 - 6.6
1941 - 8.4
1942 - 11.6
1943 - 19.3
1944 - 34.1
1945 - 7.2
Total - 87.2
Soviet Union
Rifle production (in thousands):
1940 - 1,462
1941 - 2,421
1942 - 4,049
1943 - 3,438
1944 - 2,451
1945 - 703
Total - 14,524
Machine gun production
1940 - 96
1941 - 149
1942 - 356
1943 - 458
1944 - 439
1945 - 109
Total - 1,608
Artillery (not incl mortars)
1940 - 15
1941 - 41
1942 - 128
1943 - 130
1944 - 122
1945 - 77
Total - 514
Tank and SPG production
1940 - 2.8
1941 - 6.6
1942 - 24.7
1943 - 24
1944 - 29
1945 - 22.6
Total - 109.7
Combat aircraft
1940 - 8.3
1941 - 12.4
1942 - 21.7
1943 - 29.9
1944 - 33.2
1945 - 20.9
Total - 126.4
Ratio of Soviet production to German production:
Rifle production (in thousands):
1940 - 1.1
1941 - 1.8
1942 - 3.0
1943 - 1.5
1944 - 0.9
1945 - 1.1
Total - 1.5
Machine gun production
1940 - 1.6
1941 - 1.5
1942 - 3.0
1943 - 1.7
1944 - 0.9
1945 - 1.0
Total - 1.4
Artillery (not incl mortars)
1940 - 2.4
1941 - 1.8
1942 - 3.0
1943 - 1.7
1944 - 0.9
1945 - 1.0
Total - 1.4
Tank and SPG production
1940 - 1.3
1941 - 1.7
1942 - 4.0
1943 - 2.2
1944 - 1.6
1945 - 5.1
Total - 2.4
Combat aircraft
1940 - 1.3
1941 - 1.5
1942 - 1.9
1943 - 1.5
1944 - 1.0
1945 - 2.9
Total - 1.4
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* With the exception of tanks, but that is a bit skewed by the 22,000 tanks the Soviet Union produced in 1945, compared to the 4,400 the Germans produced the first four months of 1945.