Obviously there was the Lend-Lease program, and I'm aware of a few instances where Western bombers would land in the USSR after particularly long missions, then refuel and fly back later. I'm curious if any consideration was ever given to the idea of creating an expeditionary force, particularly when the Soviet situation looked grim in '41, '42, and early '43. If there were such discussions, what prevented them from turning into action? Distance, logistical difficulty, political differences?
I'm primarily curious because it seems like an idea that would have popped up given the importance of the USSR to the Allied cause. If a similarly important ally were going through what the Soviets were at Moscow, Leningrad, and Stalingrad, today American/NATO boots would almost certainly be on the ground ASAP.
Hi, I've answered a similar question previously so I'll go ahead and link/copy my answer here, if you have any other questions though, feel free!
You're correct in that Stalin did desire a more direct involvement from the Western Allies prior to the D-Day landings, but the involvement he sought (aside from material support) from the Western Allies wasn't troops to be deployed alongside Soviet forces in the east, but the creation of a second front in Europe against the Germans.
Stalin and his emissaries were constantly pushing for the Western Allies to open a second front against the Germans in order to relieve the pressure being placed upon the Soviet Union at the Eastern Front. The landings in North Africa, and later Italy were not as favourably received by Stalin, as he felt these weren't nearly as effective as a front in Western Europe would be in helping to relieve the pressure being placed upon the Soviet Union. Though it is certainly arguable that these actions did help to relieve the pressure facing the Soviets.
Though the idea that the West was attempting to let the Soviets and Germans tire themselves out before opening a second front in Western Europe isn't by any means an unheard of theory or idea, the majority of contemporary histories I've read (as well as primary sources from major Allied Commands) seem to indicate that the Western Allies, mainly the Americans, were attempting to build their forces levels of experience and organization up, as well as chipping away at the mantle of the Nazi Empire, before initiating an invasion of Western Europe VIA a cross channel invasion.
While some plans were floated early on in the war (see Operation Sledgehammer) by American planners, the British were very insistent that a cross channel invasion in 1942 was deemed to fail, and that the first steps against fighting Nazi Germany should take place within the Mediterranean theater while both the American and UK Militaries gained enough strength and momentum to conduct the Normandy landings in 1944 with a higher probability of success.
Sources:
The Guns at Last Light: The War in Western Europe 1944-1945 by Rick Atkinson
An Army at Dawn : the War in North Africa, 1942-1943 by Rick Atkinson