Ok lets do that:
Sweden: They were very close to join Finland in Winter war against Russia. They send many volunteers to Finland to fight against Russian. Before Barbarossa they allowed transport of German units thru Sweden territory into Finland and after successful blitzkrieg in first days of Barbarossa they have been thinking to join German side. On the other side Western Allies (Britain to be precise) wanted to influence Sweden to join his side because of Iron mineral there. They also wanted to occupied Sweden because of the same reason.
*Volunteers* from Sweden went to Finland. The Swedish *government*, however, sent no troops. There were *volunteers* from Italy fighting for the Spanish Republic in the Civil War but the Italian *government* supported the Nationalists.
And, yes, I know that the Swedes allowed some supplies to be sent to the Finns.
However, do you have any evidence that the Swedish *government* was preparing to militarily intervene in the Russo-Finnish war? And, no, I do *not* mean the sort of "contingency" planning that *all* militaries do, I mean *active* planning for an *active* intervention!
As for the Western Allies and Sweden, well, consider why the Brits invaded Norway - and why the Germans did. It wasn't for Norway per se ... it was because Swedish Iron ore had to transit Norway to get to Germany. So why didn't they then try and invade *Sweden* ... too difficult, especially if the Swedes are selling you the Iron ore (if you're Germany) or for simple geography (if you're the western allies).
Turkey: You said yourself for German side. I?ll just add one true fact. Stalin demanded from Turkey to accept Russian bases near Bospor to gain gate to Mediterranean sea. He has planed to attack Turkey if they refuse. It was a very serious threat but then Barbarossa started.
Turkey is interesting. Yes, she was allied with Germany (or, more correctly, the *Ottoman Empire* was) in WW1. But Kemal Ataturk left strict instructions to his successors to NOT ally with Germany. And to say that the Turks *still* Hero worship Ataturk would be an understatement. Note also that the Brits basically *bought* the Turks, to a degree, by guaranteeing sales of all strategic materials produced by Turkey for the duration of the war. Germany ... well, economically Germany was a basket case in 1939 as far as international trade was concerned, and, well, Ataturk *had* said "don't ally with Germany!"
As for Uncle Joe ... he could threaten all he wanted, but he really had no capability of invading Turkey. Logistics, logistics, logistics make the Caucasus unsuitable for any chance of an invasion succeeding. And a maritime effort? Forget it.
Spain: You said historical facts about Hitler-Franco hard negotiations. Your statement that players would have gained nothing if Spain joined is ridiculous. What about Gibraltar ? the most important strategic point?
Spain is an interesting case. The German military did some planning for the possibility of an invasion and came up with the interesting conclusion that, because of the nature of the Spanish rail and transport net, even if they could get Spanish co-operation, they would only be able to supply a reinforced Corps or two at the pointy end (Gibraltar) with one Luftflotte in support. And even that would result *mass famine* in Spain.
The Spanish relied on imported - by sea - food during the war. Guess what happens if they go Axis? It's called "blocakde" ... no food. And their railnet was, as the Germans found, structured so that it was most efficient at moving things from ports to the hinterland, or from the hinterland to ports, *not* across country from the Pyrenees to Gib. So even if Germany had been willing and able to supply Spain's needs, she would have been, so the planners found, incapable of actually moving it into and around Spain.
Why is this important?
Guerillas. Think peninsula war. Think "Western Allies start supplying Republican remnants in a *serious* manner" ... think "bleeding ulcer" ... think hundreds of thousands of Axis troops tied down unproductively
As for Gibraltar ... well, the Germans figured they could supply a reinforced Corps. Say 4-5 divisions. And they all have to attack across a narrow neck of land around 800 yards across (or less), then up cliffs full of fortifications. Think piles of dead bodies clogging the approaches.
To make a serious attempt they will need to do more than that ... they will need serious naval support, which means the Italian Fleet must be committed and, seriously, in any RN vs. RM fight the RN wins. And the Italians get trashed.
Which means that, even if the Germans *do* manage to take Gibraltar, it will be, at best, the most awful of Pyrrhic victories. I mean, apart from the brigade or so of troops the Brits had there and some ships they lose because they destroy the Italian navy, what do the Allies lose?
Access to the Western Med? Not really, by 1941 (the earliest Operation Felix can happen, realistically) they were sending their convoys around the Cape anyway. Malta? They can push convoys through from Alexandria just as they did from the other direction just as successfully.
What do the Germans gain? A bleeding ulcer they can't defend and which the Allies can easily invade from around 1943 on.
Access for the RM to the Atlantic?
Nice, but the RM gets destroyed in the attempt to take Gib ... and even if it doesnt, well, the RM had short legs, and was not all that useful outside the Med as a result.
Worse, the Axis loses the Azores and Canaries to the Allies who use them as fleet and air bases to blockade the straits anyway ... even worse, they provide facilities for aircraft to close the mid-atlantic gap a couple of years earlier than historically, emasculating the German U-Boat fleet sooner.
Worse, with the RM, at the very least, more than decimated, it makes supplying the DAK even more problematic than it was, meaning the allies win in North Africa sooner, and which, in turn, means that they stage Torch against Spain/Portugal or against Sicily/Italy.
So, on the whole, Spain was a whole can of worms, historically speaking. In a *game*? Well, if it is seriously trying to emulate historical constraints, it should be the same. If its a complete hunk of crap, in realism terms, like HoI forex, then that's fine. But I, for one find HoI offensively stupid for that sort of reason.
It is not hard to make all this in some simple diplomacy concept and add to a game more fun parts in play. I assume that you have played long time ago expansion for Axis and Allies called World at War? It had most simplified diplomacy concept but still this concept added so much tactical and strategic options to gameplay.
Most diplomacy modules I have seen, especially for World in Flames (Days of Decision?) are crap. They do not in any way model what the real world constraints on actions taken by the participants were, and allow the most ridiculously ahistorical course of events to take place.
YMMV of course [:'(]