I remember rather recently reading a post which I cannot for the life of me find now which went into the preparedness of the German state and military, it's advantages and disadvantages over it's neighbours, what led to it's early successes and whether any European leader really thought war could be avoided. Being unable, despite a fair bit of searching, to find said post, I thought this sub to be one of the best places to ask these questions.
Also asked this on r/WarCollege but thought this sub might also be able to help on the more historical and less strategic element of the question particularly.
Also if anyone recognises/knows the post I'm talking about a link to it would be greatly appreciated.
I think the big mistake you can make when thinking about this question, is thinking about 'WW2' as a hole. What WW2 would be, was not clear in 1939. Germany in say 1938 was horrible prepared for a the World War that actually ended up happening. Only thanks to their incremental improvements could they have achieved what they did.
Looking at Germany in 1938, it is quite insane that they took a gamble and actually managed to take Czechoslovakia without a real fight. This gave them more tanks, more money (very important) and still the bigger nations were not in full arming mode. This was a huge gamble because while they could have taken on the Czech Army (eventually), but if Britain, France and Poland would have declared war then, Germany would have been gone in 1-2 years.
They were well prepared in 1939 to defeat Poland but not to take on France/Britain. The attack on Poland went of nicely and proved many of the tactics, helped them improve on all aspects for the next fight. Had France been offensively minded they could have taken out Germany, essentially the hole army was in Poland and once they came back, they were in need of major refitting. The most significant event was the alliance with the Soviets.
Now we come to what I think is the pivotal year for the WW2 we actually got. In 1940 Germany attempts to improve on their failure in WW1 and invade France once again. This time around they have the Soviets in their back supplying massive amounts of material. Thanks to this Soviet alliance Germany can put together a impressive Invasion Army. When all is said and done, I think its fair to argue that Germany in 1940 had about the same strength as France, Britain and Belgium together (on land/air in 1940). This of course suggest that they should fail, because a defender has a lot of natural advantages, specially France who had invest a lot in defense and had it as a core part of their strategy. Additionally with War production ramping up in both France, Britain and the US (because of France/British contracts).
This move was once again very bold. Had the initial attack not worked and had the war dragged on, Germany would have started to lose quite quickly. They really did not have the production and material bases to fight France and Britain, even if the Soviets are supplying. As we know however, that did not happen, France went down and Germany was dominating the continent. This victory also bound the Italians to them fully. Now they gain a lot of materials that the French have stored up (expecting a long war) and a lot of production tools as well.
The next enemy they have to take on is Britain. To fight Britain they are not really prepared all that well. The Luftwaffe is powerful but does not have a change of bombing Britain into the ground. Their Surface Navy has absolutely no change of taking on the RN. Their U-Boats are to few for victory. Their Army was of course massively more powerful, but also competently useless. Fighting in North Africa was only possible with the Italians and it was never a spot where you could really take Britain out of the war. So Germany was not prepared to fight Britain. Fighting Britain would have been a long term struggle and its unclear that they can win.
Another huge problem is that Germany has the Soviets in their backs, and its Soviets supplies that makes it possible for them to fight this war. The Soviets could at any point blockade Germany and Germany would have been toast. So going into a long term struggle with Britain while the Soviets are building up their army, demand outrages prices for their goods (such as plans for planes and chemical plants) is not a good situation.
So to solve both problems, Germany takes on the USSR. For this fight they were quite unprepared, the only hope for victory was political instability. This is the time when Germany goes from 'maybe win against Britain in a long term struggle' to 'almost impossible to win against both Britain and the USSR'. When the US joined shortly afterwards it goes to 'German and Japan are totally in over their heads and have no change of winning'.