There's a great book called the Wages of Destruction which examines the German economy before and during WW2.
Germany was a relatively poor country and the rearmament programs all but bankrupted it. Thanks to incredible amounts of economic manipulation and fortuitous gains from Austria and Czech., it managed to keep going but in the long-term, it would lose to a Franco/British alliance because their superior economic base would eventually outproduce Germany. This forced Germany to war early and was far more successful than it expected, giving it control of nearly all of Europe west of the USSR. However, Europe is relatively poor in resources and especially oil so it wasn't enough. In fact, it became a drain on German oil and coal supplies.
Germany wanted to be able to challenge the USA but it didn't have the resources to do so even with Europe. It needed the USSR. The best thing may have been the USSR joining the Axis but that would have required such a change in ideology at the top that it seems impossible for it to happen long-term. That's why it invaded.
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