Arguably is an understatement.

Perhaps you're considering only the European theater, but even that would have been significantly more challenging for Russia without the U.S. tying up (and degrading) Axis resources and manpower throughout Europe and elsewhere (e.g. the Pacific). Japan could have very well opened an eastern front for Russia.

And, it was the U.S. that forced a two front war that prevented Germany's fuller focus on Russia's western front (millions fewer troops). Not to mention U.S. logistical and material support to the Soviet Union, which may well have prevented their industrial collapse.

Even with all of this support, the fatality rates for Russia were astronomical. To this day, it boggles my mind that one nation lost ~26 million people in a single war.

Hard to imagine how they would have succeeded without the U.S.