Germany never had a chance to win the war. Many people today don’t realize just what an economic and military powerhouse the U.S. was during WWII. Once the U.S. entered the war, the final outcome was never in doubt.
For example, by 1942 the U.S. was producing more tanks in a single year than the entire number of tanks built by Germany in its entire history starting from WWI.
By 1941, Germany’s industrial infrastructure was at full capacity, and all production was devoted to creating weapons. In the U.S., on the other hand, there was still so much excess industrial capacity that not only was the country producing more arms than all other combatants combined, but it was still building more factories, meaning the gap would have grown larger.
Even so, the U.S. is the only major combatant to not suffer so much as a single quarter of recession during the course of the war. U.S. wartime production reached its peak in 1942, after which the U.S. started to scale back because it already had just an overwhelming capacity to produce weaponry. And even so, the U.S.'s percentage of production devoted to the war was by far the lowest of any major combatant. The U.S could have easily doubled or even tripled production of weaponry, given enough time.
By the end of the war, Germany had lost so many soldiers that it was drafting 14 year old boys for war. In contrast, the U.S. had only lost something like 240,000 men, out of a male population of over 100 million. So it had a huge advantage in being able to field armies as well.
When WWII started, the U.S. was severely lagging in technology, as well. Its airplanes were inferior, many of the U.S. warships were leftovers from WWI, and American tanks were no match for the German or Soviet designs. Nevertheless, by the end of the war the U.S. had the best, most advanced weaponry of any of the combatants overall, although in a few areas they still lagged a bit. But given a longer war, the U.S. would have rapidly passed by the other powers, as it did after the war.
And while all other combatants were having their infrastructure bombed, the U.S.'s was completely intact, and would have remained so even if the war had lasted another ten years.
So what would have happened if Germany didn’t invade the Soviet Union? Here’s my guess: The war would have lasted until maybe 1946 or 1947. The U.S. and other allies would have lost maybe twice as many soldiers, but then the atomic bomb would have ended the war anyway. But the Soviet Union would not have captured eastern Europe, the eastern territory it grabbed in the later part of the war, or east Germany.
The cold war would have been very different. There probably wouldn’t have been a Korean war, and the Soviet Union would have wound up much weaker. In addition, without the horrible losses of World War II, it would have been interesting to see if Stalin could have maintained his brutal grip.
No matter what the result, the world would look very, very different today. But the U.S. would still be the big power in the world.