Winterz 说:
Lol, you as a Finnish(don't they teach history in school, kid?) should know better that you would be the first ones to be invaded by the soviets.

I love,
love how you talk as if the Soviets invading Finland was a hypothetical scenario. You do know that Finland was invaded by the Soviets, right? For quite some time, too.
As for the American and British presence on the Western Front being critical to the Soviets' success, the Sovs were already well into Poland before the invasion of Normandy happened. D-Day was June 44. The Battle of Kursk was finished 11 months prior. Stalingrad had been won approximately six months before that. And you are asserting that the Normandy invasion, which wasn't to come for another year, was critical to the success of the Russians? The US troops, at that time, were invading Italy, not France, and the Germans redirected comparatively few troops to Italy. Further, a good portion of the troops stationed in France would not have been able to be diverted, as the French Resistance was actually posing a fairly credible threat to the Germans at the time. While the FFI had lost some fairly important battles, they were still able to liberate Corsica in '43 and the Nazis were never aware of just how tenuous the FFI's existence was. The Germans never would have left France abandoned. Even if they had wanted to, they couldn't. You can only get so much food, oil, and ammo to one location at a time. Adding 7m to the Eastern front was going to add logistical nightmares to a corps already plagued with same. Troops need support and supply lines, which do not always have the ability to swell forever.
Your idea that without D-Day, the Germans could have invaded Turkey and turned a flank through the Caucasus is simply bizarre. First, the Caucasus are incredibly mountainous, not something rapidly moving tank columns do particularly well with. Second, the British, as well as Afghani irregulars, remained in Afghanistan at the time, well able to flank the possible flank. Third, the British and other Allied forces (conspicuously devoid of US assistance) had won at El Alamein in the late fall of 1942, giving two decent starting points for a potential invasion of Turkey should it be necessary.
A year before D-Day, both the British and Soviets had beaten the Nazis on their own terms. The Blitzkrieg had been defeated on two fronts, Middle-Eastern oil had been secured, the Battles of the Atlantic and Britain had been won, and the Allies were counterattacking on all fronts.
Now, that is not to say that the US was completely worthless. Financial assistance and materiel support were actually very important to the success of both the Soviets and the UK. The strategic bombing of German and French factories, almost entirely performed by the US superbombers, also contributed to a major drain on Germany's finances and production capabilities. However, this did not start until mid 1942, so had no appreciable affect on the Eastern Front.
Do I think the US was entirely worthless? No. However, their main contribution to the war in Europe was to supply arms to the Russians and the UK. When they did enter the war in a practical, military manner, they largely served to hasten the already occurring demise of the Nazi regime.
And, as pointed out, set the stage for a very different antebellum period, but that is neither here nor there.