This thread has jumped around a bit with regard to World War Two in Europe, so I'll try to inject some basics for consideration without getting to long and in depth.
First let's look at Russia/USSR ~ the "Eastern Front".
Prior astounding success of "Blitzkrieg" against Poland and France, along with seeming adequate containment of Britain/UK led Hitler and many of his General Staff to think they could defeat Russia/USSR in a few months with "Blitzkrieg". As it was, Russia was adept at slow retreat and trading land/space for time to recover it's manpower and material/weapon resources while giving the German's a costly gain. Winter 1941 hit sooner than usual and the Germans were not expecting or prepared to deal with such, their troops not having clothing and equipment to deal with harsh "General Winter" in defense of Russia.
As it was, German forces made it through that first Winter and made even greater gains of land and Russian turf in 1942, but still not enough to defeat Russia.
Had it been just Germany against Russia, Germany might have prevailed. But Allied (mostly USA) Lend-Lease helped to barely tip the scales/odds to Russia's favor. With the Ukraine and most of Western Russia, where that nation's main food supply was grown, now in German hands and/or site of back-n-forth battles, Russia was facing serious threats of starvation by end of 1942 and thru 1943. This is where USA Lend-Lease in form of food (wheat and 'spam') was a major saver to Russia, keeping the nation and it's armies feed during those first critical years (and beyond actually).
Other aspects of mostly USA Lend-Lease to Russia are what would amount to about 80+% of motor transport ~ jeeps, trucks- 4x4 & 6x6, half-tracks along with significant amounts of railroad locomotives, flat and box-cars, and steel rails. Also about 20% of aircraft, fighter and light~medium bombers, and about 10+% of AFV~tanks. Another overlooked and under-appreciated item was about 80-90% of radio equipment. Also a significant amount of medical/first-aid supplies.
To the Western Allies ~ USA and UK/Commonwealth ~ keeping Russia in the fight and tying up about 80% of the German army/divisions and significant parts of their air force was essential to giving a chance for success in the Western part of the European (ETO) war/campaigns.
With Russia involved and gaining an edge against the Germans, the War was looking winable within a few years. Russia out of the war and it could go for decades, if not be lost all together. Allied Lend-Lease was the factor to help tip the scales to Allied favor against Nazi Germany.
