Dear 100 Hour Board,
I am a recently returned missionary who served in Russia. Could Soviet forces have defeated the Nazi forces in Europe without the US and her support? By the time the US entered the European Theater, was Russia already marching to Berlin? I'd like the facts!
-A more dedicated Patriot
I don't think you can limit the question to when the U.S. entered the European Theater. I think you have to take U.S. participation as a whole. If Japan had not attacked Pearl Harbor, bringing the United States into the war, then there is a good chance Japan would have made a move against Russia in Siberia. Had they done so, Russia would not have been able to take the risk (which they did) of pulling out 30 divisions of well trained and equipped Siberian troops to defend Moscow and lead the counter-offensive in November/December 1941. Had Japan made a strong offensive move in Siberia in 1941 I think there is a good chance that the German army could have taken, or at least laid siege to, Moscow in the spring of 1942. The deciding point, really, would probably have been how much of the German army was left standing after the brutal Russian winter. If Moscow had fallen and the German advance resumed in spring 1942 with a Japanese offensive in the East then it seems unlikely that Russia would have survived.
But let's suppose Hitler hadn't declared war on the U.S. after Pearl Harbor (no one can seem to figure out why he did it) and that the U.S. entered the war only in the Pacific Theater and had no participation in the European Theater (except the mass quantities of munitions we were selling Great Britain). The aforementioned counter-offensive that ended the Battle of Moscow was successful, though costly, and gave Russia the time it needed to fully mobilize its economy. In Summer 1942 Germany ran another offensive against Russia but was halted at the Battle of Stalingrad. These two battles combined are generally considered the turning point of the Eastern Front. After this time the German army generally only lost ground to advancing Soviet forces.
When we talk about the U.S. entering the European Theater I'm sure most people think of D-Day and the Normandy landings. D-Day wasn't until June 1944. From summer 1942 until summer 1944 Soviet forces were successfully advancing against Germany and making good progress. This would suggest that the answer to your question is "yes". But, while the U.S. hadn't entered France at this point we were still involved in North Africa drawing away German troops, supplies, and focus. And Italy was invaded in 1943. Even before the U.S. joined, Great Britain had been running a North Africa campaign, but the needed commitment of German forces surely increased when the U.S. arrived. Could Germany have repositioned troops from North Africa to the Eastern Front to successfully subdue Russia if the U.S. hadn't joined? There's no way to know for sure, but it would probably have been a toss up. The evidence suggests that Russia barely survived the winter of 1941-1942 and had German troops been better equipped for winter with more troops and fewer distractions in North Africa then Moscow probably would have fallen. And had Moscow fallen it would have been difficult for Russia to organize a coherent response with the remainder of their forces.
In summary, had the U.S. not entered World War II at all it seems likely that Russia would have fallen from a two-front war with Germany and Japan. Had the U.S. only entered the Pacific Theater against Japan, Russia would have faced a much stronger and more focused German army that would have been very difficult to overcome.
-Curious Physics Minor
*More accurate terms in many places would be the Axis, Soviet Union, and Allies but I used country names for convenience.