~Whoa, slow down and decide if you want to ask a question or to answer one.
Germany and Russia were not allies. They signed the Molotov Ribbentrop Pact in 1939, but that was a non-aggression treaty by which they agreed on how Poland, Scandinavia and parts of Eastern Europe would be divided. The were natural and bitter enemies and war between them was inevitable. Molotov Ribbentrop just gave them some breathing room to eat up their neighbors without having to worry about each other, with each side knowing the other was preparing to attack at some time. Hitler's greatest coup, through Goebbels, Himmler and Heydrich, was to convince Stalin that his generals were planning to overthrow him. That led to the Great Purge. Hitler's greatest blunder was to underestimate the ability and will to fight of the Russian people, the ability of the new corps of officers, the strategical mind of Stalin and the quality of Soviet weaponry and their ability to produce them.
Stalin's decision to engage in a fighting retreat in the early days of the war until he could move his factories behind the Urals and get them into full production, especially of tanks, artillery and warplanes, and then to make his stand on ground of his choosing at which he had constructed his defenses (Stalingrad, Leningrad and Moscow) was nothing less than brilliant and sounded the death knell of the Third Reich. The winter didn't beat the Germans - superior forces (in will, ability and numbers), superior weapons and superior strategy did.
Nobody with a lick of sense and even an elementary knowledge of history believes the US joined the war because of Nazi treatment of the Jews or the other 12 million Nazi victims who died in the camps and at the hands of the SS. The SS St Louis, the Evian Conference and US Jewish immigration policies are all one needs to establish that basic tidbit.
Yeah, the seeds of WWII were sown by the Treaty of Versailles, but Poland was a minor grievance at most. The Germans had far more serious gripes. The British and French signed a Treaty with Poland whereby they promised aid if Germany attacked. In September, 1939, they honored the Treaty by declaring war after Fall Weiss was launched, but they then sat on their hands for the six month Sitzkrieg, or Phoney War. Only after the Low Countries and France were attacked did they actually engage in the fighting.
British casualties on all fronts during the entire war amounted to 320,000 KIA and 62,000 civilian dead. Had Hitler concentrated on the British Isles before launching Barbarossa, Britain would have fallen. Hitler lacked the landing craft and logistical support to pursue a ground war in England and the window of opportunity to go after Stalin before Stalin came after him was fast closing. The UK was essentially put on hold while the more important conflict in the east was prosecuted. Then the Germans had to bail out the Italians in Africa, opening third front. All this before the US fired the first shot (which they wouldn't do until Operation Torch in Africa in November 1942).
Germany was helping China against Japan in the Second Sino-Japanese War. So was the UK and the US. The US claimed neutrality in both theaters, but chose sides and helped the enemies of Germany and Japan with money, arms, supplies and equipment and unofficial troops and advisers. Continuing US violations of neutrality caused Germany and Japan (with Italy) to sign the Tripartite Pact in 1940, moving Germany from the Chinese camp to the Japanese. The pact was intended to force the US to live up to its obligations as a neutral. It didn't work. The US supported Ho Chi Minh and the Viet Minh in French Indochiana, violated the Washington Naval Treaty by building of bases on Guam, Midway, Wake, Pearl and the Philippines, created the USAFFE and its air win and planted it on Luzon, at Subic Bay and at Clark Field, seized all Japanese assets in the US and increased already onerous embargoes against Japan. The Hull Note ultimatum was the last straw. The result was Pearl Harbor, which the Japanese hoped (half-heartedly) would keep the US out of the war. Their idea was to strike a comprehensive crippling blow that would render the US incapable of fighting and bring about a quick treaty and the relief of the embargoes and return of assets that diplomacy had failed to deliver. They knew they couldn't win a prolonged war with the US and had no intentions of invading the US. Had the carriers been in port, had Genda been allowed to launch the third wave against the oil storage facilities, the drydocks and maintenance yards, had Nugumo gone after Midway on his way home, the plan might have worked.
Meanwhile, FDR was at the same time moving from the Neutrality Acts through Boats for Bases to Lend Lease, with the coup de grace being FDR's announcement on 9-11-41 that he had issued orders to the navy to attack German warships on sight without provocation if they were spotted in American defensive waters - which he claimed extended from US shores to the coast of Ireland. Increasingly aggressive US behavior on the European front inevitably brought the US into the war.
US contribution in Europe, militarily, was minor. The US and RAF bombing of Germany's industrial base and transportation infrastructure helped the cause but the war was won by the Red Army. US losses in both theaters during the entire war were less than 300,000. The Soviets lost more than that in 3 weeks during Operation Mars alone (Mars failed mostly because the Russian Winter crippled the Red Army to the great advantage of the Germans) and suffered 10 million KIA (with another 15 million or so civilians) during the war. Normandy was a cake walk compared to Smolensk or Khursk or Kharkov, and a holiday compared to Stalingrad. As Churchill told Roosevelt, the only force on the planet that stood any chance of winning a war against the Wehrmacht on the continent was the Red Army. Stalin proved Winnie correct. Regardless of what was happening on the Western Front, after Barbarossa was repelled and Operations Saturn, Uranus and Mars turned the tide at Stalingrad and in the east, Soviet tanks were going to roll into Berlin.
Lend Lease helped to be sure. The Soviets loved the trucks. However, US tanks, planes and artillery were they worst fielded by any of the major belligerents. Bad planes are better than none, but the British and Soviets preferred to fly their own simply because they were so superior to US counterparts and next to a T-34 a Sherman tank was a death trap of a tin can.
The team that built the bomb was an international group, including Danes, Norwegians, Canadians, British, Italians, Germans, Poles, Americans and others. The German program was ahead of the Manhattan Project until it was placed on secondary priority status by Hitler and possibly sabotaged by Heisenberg himself. The bomb was the product of decades of work in physics with the discoveries and advances being piggybacked one atop the other. It started in France with the Curies, wending its way through Rutherford, Cockcroft, Chadwick, Hann and Strassman, was given a boost by Einstein and culminated with Frisch, Meitner, Bohr, Rabi, Lamb and Fermi.
According to the US Strategic Bombing Survey, a study commissioned immediately after the war by and for the US government, the bombs dropped on Hiroshima and Nagasaki did not bring about the surrender. Eisenhower, Chief of Staff Leahy, Nimitz, MacArthur and others recommended against using the weapons as unnecessary. The Japanese had been sending out peace feelers through back channels for months and were ready to surrender. The convention bombing and firebombs which destroyed 67 Japanese cities and killed up to 2 million Japanese civilians during spring and early summer of '45 did far more damage than Fat Man and Little Boy. The nukes were not targeted at military targets and none of the next 11 were going to be. Top military planners pleaded that the bombs be saved until they could do some good for the planned invasion, or at least be dropped on significant military targets or troop concentrations but they were not heeded. Operation Downfall, the invasion of Japan, wasn't scheduled until November, 1945, and wasn't going to happen anywhere near any of the planned nuclear strikes. The troops expected to meet the invasion were not targeted either. The Bombing Survey concluded that the embargo, the starvation of the Japanese people and the conventional bombing would have brought about the surrender before Downfall set off. Japan had no navy left, very little air force, her best ground troops were long since dead and the green replacements were slowly starving and being sapped of their will and ability to fight. 60% of the civilian population favored an end to the war and the country was under martial law for fear of a popular revolt and yet another coup attempt. Japan's industrial base was in ruins and she had no access to raw materials to produce weapons or oil to power them. Japanese government and military officials confirm that the peace movement, which had already toppled Tojo's cabinet, was in the majority and at the very doorstep of surrender before the bombs hit. Harry Truman was aware of all of this but Cordell Hull convinced him to use the weapons to appease the voters.
The US Navy did contribute greatly to the end of the war with Japan, but don't discount the job the Chinese did. The bulk of the fighting happened on land in China, Manchuria and Indochina. The Viet Minh held their own in Indochina and in time the Chinese would have, at the very least, fought Japan to a stalemate. Rumor has it that the Aussies and Brits were kinda busy in the Pacific, Indonesia and Asia as well.