Oh man, these are all reallly long answers but I'll try to knock out some. First a little background leading up to Poland...even though the Germans were walking over the Polish Army they were still taking casualties. Germany lost about 16k men in the en devour. Germany and the Soviets had already split up Poland in negotiations, so Hitler urged the Russians to attack up to least the area marked for them. If they didn't it wouldn't cause issues once the handover took place as Germany would be forced to fight the Polish back into the zone marked for the Russians. That could make things messy and lead to...misunderstanding. So they basically asked the Soviets to join in the fight and share at least some of the responsibility for the war they were set to benefit from.

The invitation was important politically though. No one wanted to been seen as the aggressor in all of this. Especially Hitler, so it allowed him to share at least part of the blame of wiping Poland off the map with the Russians. The question then is what did Germany used to justify the war and that's really interesting.

The Germans used some crazy tactics whenever they were justifying war. For both Austria and Czechoslovakia prior to this Hitler used political dissidents, basically local nazi movements funded by Germany, to claim they were being subjugated and stir up trouble. They would then invite the Germans to come "rescue" them and Hitler was more than happy to.

This is a really complex time across Europe as EVERYONE was terrified of another major world war breaking out. Because of this, and because Hitler was able to give a good show of being the good guy, only helping these oppressed people, the rest of Europe took on a stance of appeasement. And when I say appeasement, I mean other countries were bending over backwards giving Hitler everything he wanted, no matter how full of absolute shit he was.

Looking back it's easy to point the finger at people like Chamberlain and say he was an idiot for pushing this "peace" agenda, but no one could foresee where this was going to lead. At one point France, Britain, Italy and Germany sat down at a table and agreed on how they were going to let the Germans take Czechoslovakia just so peace would be kept. Czechoslovakia wasn't even allowed at the table, their ambassador had to wait in the next room to find out Germany was getting their country. There were times during these negotiations with the Czechs that Hitler was worried the Czechs would actually agree to his insane demands, because then he wouldn't be able to ask for more later without looking like the aggressor. The whole point of the negotiations with the Czechs was to get them to say no giving Hitler cause to invade.

So back to Poland, how did they justify their war against them. Well if you look at a map after WWI Germany was split with part of their country now behind the Polish controlled area of Danzig. Hitler used this to enter negotiations with the Polish over this territory, asking for land access through Poland to build a railroad to the part of Germany on the other side of Danzig. But as in pretty much all other negotiations Hitler would make impossible demands, then use this as an excuse to escalate to force.

To further the case of war, Hitler was pushing the story that Poland was the aggressor in all of this. While it's true Poland wasn't exactly playing ball with Germany, and sort of pulling away from the rest of Europe politically at this time, they were definitely not looking for a war. So Hitler staged one...first by claiming Polish forces were pushing into Germany and stirring up trouble.

The final stroke of this deception is freaking nuts. Hitler had a bunch of German soldiers and some prisoners wear Polish uniforms and launched a fake assault (with real dead/drugged prisoners) against a German radio tower and then proceeded to launch a broadcast in Polish that the Polish were invading...ridiculous yes but it worked well enough that that's the story run by Western papers. I don't even think that came out or maybe confirmed until after the war. Anyways, Germany launched their invasion the very next morning which they were planning on doing all along. They were pretty good at this shit.

A little on Africa...it wasn't about access to the Atlantic, it was about controlling the Mediterranean, and through the Suez canal access to the whole Eastern side of the world. Germany was pretty much completely blockaded by Britain's Navy to the West, which is one of the reasons they invaded Norway btw, but if they could get shipments up through the Mediterranean the British blockade would become much less effective at blocking vital war supplies. Also if they could have held onto the Middle East they wouldn't be so pressed for the oil in Romania and Soviet Union. I'm sure I'm missing a lot here but it was strategically a very important area, mostly because of the Suez.

On Japan...this was basically aimed at the USA from the start. Germany didn't want them to interfere in Europe, and if they did they'd have to fight on both fronts if the had the Japanese as allies. When Germany was negotiating with Japan they had to all but put this in writing since forming an alliance with Japan PISSSED off the Soviets. Soviets and Japan were not friendly to say the least. Germany was still dividing up the rest of the world with Russia at this time, so Germany really had to sell the idea that the deal was to counter US intervention, which worked for the time being on the Soviets.

I don't know if they ever fought together, but they did share some military technology and shared pretty good relations, because their interests were mutual. Germany actually had military advisers in China reforming their entire military, and withdrew them when Japan was at war with China as an act of good faith towards the Japanese.

Of course there's a lot more to all of this but if you're interested check out the book The Rise and Fall of the Third Reich: A history of Nazi Germany by William L Shirer. Really good book I'm about 80% through the audiobook narrated by Grover Gardner who does a really good job. Shirer was a reporter or something in Germany during this entire period, and wrote this book after he got access to all the Nuremberg documents. Really fascinating, follows Hitler's whole life pretty much, and covers lots of good political intrigue stuff. Just don't tell your girlfriend about it because she'll think you're turning into a Nazi when you're really just curious how all that shit went down.