Germany in 1939 relied on sea trade for many essential war supplies. Oil had to be imported from the US and other countries by ship. Germany was also not self-sufficient in food supplies and other goods.
Britain's declaration of war against Germany in September 1939 meant that Germany was blockaded by the Royal Navy. There was no realistic prospect of challenging the Royal Navy in open battle, and Germany could not break the blockade. Germany's pre-war stockpile of oil was diminishing. The only remaining major market open to Germany was the USSR.
It was this situation that prompted Hitler to invade the USSR. Germany was completely reliant on Soviet supplies of oil and other essentials due to the blockade. This put Stalin in a position where he could potentially cripple Germany simply by shutting off supply lines. To Hitler, this was an unacceptable risk. By the time Barbarossa was launched in June 1941, it was estimated that Germany had sufficient oil reserves to continue the war for three months.
Conquest in France and the USSR combined with synthetic oil production and an all out extraction of oil from Romania allowed Germany to drag out the war for longer, but there was never enough oil. It is the reason why in 1942 Germany did not attempt to take Moscow but instead launched Fall Blau towards Stalingrad and the oil fields of the Caucasus. And it is one of the main reasons why the sixth army could not be supplied at Stalingrad.
Did Germany lose the war in September 1939, by getting into a war against Britain which Germany could not win? It seems to me that every step of the war from here on (war against USSR, declaring war on the USA) stems from the original miscalculation in September 1939.
What do you think? Was Germany's defeat inevitable from the start?
Assuming that, say in October 39, after realizing that UK and France will not budge on the issue of blockade, Hitler decides not to rely on the shaky possibility of future imports of fuel from the west and starts an accelerated mass program of construction of synthetic fuel plants using resources (coal and labour in the first place) from the occupied territories, such as Polish Silesia and western Czechoslovakia. After initiating the blockade, Germany lost access to around 4 million tons of oil per year imported from the west. In the end of the war Germany produced over 7 million of tons per year from synthetic plants, so a lot of work had been done here but it started later than in this ATL. So let's assume that Germany gives synthetic fuel strategic priority and starts a crash construction in October 39 of enough plants to compensate for the lost 4 mio tons a year and adjust vehicle (especially truck) production accordingly (let's they are able to overcome the rubber problems).
Assuming that the first 1 mio/year capacity is available in two years' time, and it would all be used for military purposes only, it would mean extra 80.000 tons/month for Barbarossa between October and December 41. That would probably keep in motion extra 15-20 panzer and motorised divisions. What the effect would be is a big question, perhaps greater mobility in front of Moscow or a parallel attack by the Army Group South.
Continuing with this, let's assume that in Spring 42, already 2.5 mio of the 4 mio/year extra capacity (i.e. extra 200.000 tons of fuel/month) would be ready and available for the summer 1942 campaign. Again, had it all been used for ground warfare, that could keep twice the number of German panzer and motorised divisions in motion (in Barbarossa and summer 42 Germany had 30-35 panzer and motorized divisions, though since mid Barbarossa they never had enough fuel to run them at the same time). Assuming that the vehicle production would keep up the step with fuel production, that would mean that in summer 42 the German army could attack with roughly the same strength as in early Barbarossa, at least with both army groups center and south, not just south. What effect it would have on capturing Maikop, Grozny oil fields and capturing or at least disabling the Holly Grail of the eastern front - the Baku oil fields (altogether Maikop, Grozny, and in the first place Baku, produced 80-90 percent of Soviet oil, had Baku oil been lost for the Soviet Union, the SU would barely had enough fuel to operate its tractors in agriculture, let alone fight Germany), I can't say. But it would certainly be interesting to speculate about.