From my understanding, the Germans made a lot of mistakes and miscalculations that eventually led to their defeat. However, Germany, with all of its weaknesses managed to take over France and were heavily suppressing the British. This was before the bombing of Pearl Harbor and USA entry into WWII. IIRC the USA was already proving support to the Allies through the Land-Lease act. After their formal entry, they contributed vast amounts of resources and manpower and collaborated with the Allies on military strategies. So far, it looks like the Germans lost mainly because the USA got involved. How important of a reason was the entry of the USA in contributing to Germany's defeat? Would Germany have won WWII if the USA never got involved? How significant are the other factors like the strength of Stalin's Red Army, the resistance movements going on in Nazi occupied territories or the weaknesses of Germany itself?
(I hope I provided enough information, I don't want this to sound like a 'what if..' question)
In my answer I'll focus on purely the military aspect since I think this is what you're after.
American military involvement in WWII really ramped up in the middle of 1940, this can be seen with the addition of American designed aircraft like the P-51A and P-51B into the RAF and the Destroyers for Bases Agreement. An agreement which allowed for 50 US Destroyers to be transferred under Royal Navy control in return for land in many of Britain's colonies like Jamaica and Newfoundland.
Further agreements with the USSR to provide military vehicles like jeeps, tanks, and the famous P-39 Airacobra shredded any claim of US neutrality in the war.
The first large scale combat between American and Axis forces was Operation Torch to invade Morocco and Algeria and regain control of the Mediterranean Sea.
Although most of the heavy fighting on the ground was encountered by the British, the American forces proved invaluable especially US Naval power in the Battle of Casablanca
This set the scene for the remainder of the European theatre. American involvement was needed to secure victory as British and Canadian forces were simply not enough to defeat German forces. American involvement steadily grew and grew and by the Invasion of Normandy in 1944, it was clear that the US lead the Allied forces in Europe.
Reasons for this are mainly the huge amount of manpower and a massive economy capable of huge export. Homeland America was also never threatened so the amount of industry was allowed to could grow steadily.
I could go deeper into the late war aspect if you want (Especially as I probably know more about that than the early war aspects) but it would be just expanding upon the points I've already made.