Well to a degree we need to better lay out what the actual question is.

Your title says WW2, but your text only asks about Germany OP.

Are you only interested in the European theater, Germany itself, Italy and the minor Axis Powers excluded? What about just actions on the field of battle, just what occurred fighting Germany in, over, and around Europe itself?

Are you looking for how various nations contributed to the global Allied efforts to win the war it is your inquiry more narrow OP?

America and Russia (the USSR) both contributed heavily, but served in different arenas and in different ways. In terms of numbers of casualties, the USSR's sacrifice far outweighs that of the United States. American casualties in WWII are estimated at 400,000, with little to no civilian deaths, while Soviet casualties total 11,000,000 with almost an equal amount of civilian deaths. (1) Early on in the war, before the United States had entered on the side of the Allies, she was already proving very helpful through the Lend-Lease program. This program provided vital supplies to both Britain and the Soviet Union, who were both struggling to gain the upper hand against Germany. Specifically, America supplied the Soviet Union with trucks, tanks, heavy metals, fuel, explosives, and railroad equipment. (2) In total, American aid to the USSR totaled $11 billion dollars, including a fleet of 400,000 trucks. (3) These trucks and other vehicles proved especially vital, as it allowed Soviet supply convoys to keep up with advancing troops, while Germany relied primarily on horse-drawn supply convoys throughout the war. One reason it is difficult to compare the roles of the United States and the USSR is that they fought the same enemy, but in very different arenas. The United States fought Germany in the Western Front, landing at Normandy and in Italy. While the Western Front is widely celebrated in popular culture and films about D-Day, etc. (and rightfully so, no detraction intended), the Eastern Front was on a completely different scale. Thousands of tanks, millions of men, and multiple entire armies fought on a front that was thousands of miles long. For example, from July to August 1943, the Soviets suffered an estimated 112,000 killed and 317,000 wounded in the offensives, counteroffensives, and defensive actions as a part of Operation Kutuzov, which included the Battle of Kursk. (4). In comparison, the Americans suffered roughly 29,000 killed in action and 104,000 wounded in the Normandy landings. (5). I am no professional historian by any means, and these statistics are based on personal, limited research into this topic. However, there are a few points of "analysis" I would like to add.

1. Answering a question like this can be difficult, as it is impossible to quantify how much of a role a certain group played in a war. While the Soviets did suffer far more casualties than the Americas, casualties aren't the only way to measure a group's involvement.
2. America's industrial might played a large role in the war, especially helping supply Britain and the Soviet Union. Again, it is difficult to compare with the Soviet Union's involvement.
3. All of the Allies contributed and sacrificed greatly to defeat Germany. Arguing whether or not the Allies could have prevailed against the Nazis without the Soviet Union or without the United States is conjecture at best, and is impossible to ascertain.

I'm not sure if I answered your question or not, or confused you, or perhaps made you more confused. Like I stated previously, I'm no professional historian, just an individual interested in WWII military history. Hopefully I did provide some useful information, and hopefully some of the AskHistorians moderators/experts will give a better answer than myself.

Sources: (1) http://www.eisenhowerinstitute.org/about/living_history/wwii_soviet_experience.dot

(2) Weeks, Albert L. (2004) "Russia's Life-Saver: Lend-Lease Aid to the USSR in World War II."

(3) "World War II: The War against Germany and Italy." U.S. Army Center of Military History.

(4) Glantz, David M.; Orenstein, Harold S. (1999). The Battle for Kursk 1943: The Soviet General Staff Study.

(5). Pogue, Forrest C. (1954). "Chapter XIV: The Pursuit Stops Short of the Rhine". United States Army in World War II: European Theater of Operations: The Supreme Command.

More can be added, but I have written extensively about Lend-Lease and comparison of US-Soviet produtcion before, which you can find here, and might be of interest.