Under better leadership, it could have been a very different war. If Hitler hadn't foolishly attacked Russia forcing the Country to fight a costly two front war it would have been difficult, maybe to costly, to invade germany. That was Hitler's biggest mistake IMO. Remember, Russia didn't want anything to do with the war and signed a nonaggression treaty with Hitler...Hitler broke that treaty basically because he didn't want to lose momentum after the attacks on the British mainland proved feeble. If he had concentrated all his efforts on England and developed a stronger navy quickly it would have been very different. Without England as a jumpoff point the United States would have a very, very hard time invading.
Also, Hitler foolishly squandered resources on the 'final solution' even when he was losing ground in Russia. The atomic weapons program may have been more successful if he hadn't spread thin all of his best scientists over tons of different 'super weapons' programs. The Nazi's were in development of all kinds of crazy things...a submarine that could launch a plane for instance. How useful is one plane? The super weapons program clearly had some success with the V2 rocket, but if they'd focused on developing just the atomic bomb and the New York missile, and didn't squander all their resources fighting Russia they very well could have beaten us. But those are all what ifs.
Yes, we supplied the Russians...but its the Russians that really won the war against Germany IMO, not us. It was Soviet blood all over those fields. It was the Soviets that suffered massive loses agains the Germans...never giving up, even when they were starving and cut off. The Germans underestimated them, and overestimated their ability to cover that much ground with only that many men. The Soviets had a huge army, and the blitzkreig tactics didn't work against them as well. The Germans would try to break through the lines only to find another line, an another line behind it.
The war in the pacific actually went better than planned for the US because they won a few early, lucky victories. It was still a brutal war and I would have much rather been fighting the Germans than the Japanese. Originally, we just wanted to hold them off while we got Europe under control...but ended up more successful than expected.