I know that you guys are probably sick of ww2 threads by now, but as a not-historian who has always been curious about this I have nowhere else to go.
Why was it not enough for Hitler to conquer France and oil-rich parts of Africa? Britain was more or less isolated (and unlikely to try anything w/o American troops), and he had a deal with Stalin. Why bother with those two countries at all? Was it even a rational decision or emotional one? The question might appear obvious to you, but it isn't at all to me.
Looking forward to your answers.
Hitler wanted Germany to be totally food sufficient. He had a dream of moving Germans into conquered areas and giving them large parcels of land to farm.
The problem with France was that it was full of French people who Hitler viewed as being almost on par with Aryan Germans. The Soviet Union on the other hand was full of people that Hitler viewed as subhuman and to top it all off it was covered in wide open plains of good farmland that wasn't even being used. The plan was to conquer the western USSR, divide it up into farm land and starve and work to death millions upon millions of its inhabitants. This wasn't some spur of the moment decision. Everything Hitler did since being made Reichskanzler (Reich Chancellor) in 1933 was to make Germany ready for a war with the USSR. The Non Aggression Pact he made with the USSR was to give Germany some breathing room while he prepared.
A good book with covers this and many other events in Hitler's life is "Hitler" by Ian Kershaw. Ian Kershaw is widely regarded as one of the best historians on both Hitler and the Third Reich.
Edit - One thing to remember is that France and the United Kingdom went to war against Germany when Germany invaded Poland whose independence they both guaranteed. Hitler had been hoping that they would stay out of his attacks in the east.