I will present now what I was thinking of originally responding with, I just procrastinated.
While many rue that Hollywood won the war many times over, there are some key things to note:
In the 1930s there had been some geology reports stating that there may have been significant oil reserves in the middle east. If Hitler could control North Africa, which would also allow for easier defence of the European south, that oil could be appropriated. Germany had none of its own oil, except for a synthetic made from coal, and relied heavily on Romania, that only had so much. Part of the reason Stalingrad was such an important target was that beyond to the southeast was Caucusus/Caspian sea area oil, which Germany never succeeded in taking.
The United States was part of the military force that eventually turfed the Axis out of Africa, so that is one important reason the US was good to have in the fight.
In the Pacific, Japanese Admiral Yamammoto’s carrier forces implemented a significant strike against American forces at Pearl Harbour. Hitler Might not have known immediately after when he declared war that the strike was a limited success. All of the American aircraft carriers were at sea and unaffected, and some of the downed ships were floated into drydock repairs and later saw action, the harbour was only so deep. Hitler didn’t predict the American cunning at the battle of Midway 6 months later. Codebreakers got the better of the only so good mathematical basis for Japanese communications and sent a fake message to guage their response, thus confirming their correct knowledge to decrypt.
Japan was conquering the overseas colonies of the same countries against Germany in Europe. Japanese troops were swift to take the Commonwealth garrisons at Hong Kong and Singapore, and this affected Britain economically. Japan was also leaning towards Australia and New Zealand, both resource countries loyal to England. To have captured them for themselves would have empowered Japan and weakened England. The American garrison in the Phillippines and two British Capital ships were taken out shortly after Pearl Harbour. Germany had therefore reason to think initially the Japanese would hammer at the US significantly.
The US also was a part of the Battle of the Atlantic. Even before joining conflict the US had the lend lease deal to supply England with destroyers, and then with war got in on it themselves. An American engineer came up with the hedge hog mortar, an effective anti submarine weapon that left the guesswork out of whether a U-boat had been sunk.
Germany was developing rockets. they were able to hit England with the V-2 that could not be intercepted once launched. they were planning the V-3 “New York” rocket. They were also trying to develop their own nuclear weapon, if the Americans hadn’t been helping chop them down they would have had more time.