Back to the original question, the choice of what countries to include varies depending on whether you are trying to instill some historical accuracy or simply trying to create an extremely playable game that has a wider appeal than managng the whole military-industrial-technology-political complex that is the theme of other strategy games. There was some outrage when Germany invaded neutral Belgium in WWI, that is missing from the boardgame. Germany did not take Holland because of the impact that would have had on other neutrals, the men it would have tied up, and the relatively little marginal production and manpower that would have been gained. France would never have considered trying to take over Spain, as long as it stayed neutral. Thus Paradox should consider, besides producing a version of the classic game with good enough AI to supplement for any missing powers, a version that includes some significant disadvantages of taking over neutrals. Methods could include making the neutral or conquered supply centers have half the value of a home supply center, as well as factoring it into the AI diplomatic interactions, sort of like EU2 bad boy points. Having a player for Spain, when the optimal strategy is to stay and remain neutral is questionable, although I know of some people would write AARs about how they won as Spain. Having it be handled by AI would acceptable in some enhanced variant that went beyond the simple suggestions above.
Many historians (and not just American ones) feel that the entry of the US into World War I was decisive. Germany had knocked Russia out of the war, and while both the German and Anglo-French armies were significantly worn down, the stalemate with the front lines well into France could have led to much more favorable peace resolution than what was achieved at Versailles, had the arrival of the US troops not tipped the balance. The support of the British Empire are implicitly modeled in giving England three supply centers, but the arrival of the yanks is not. Again, this could be a minor variant, modeled somewhat abstractly. If Germany does not employ submarine warfare, then the British Empire supplies more resources (the Scottish supply center) to England, or if German did unleash the U-Boats, then eventually the Americans get angry and England gets two extra units.
The scale of the units in Europe being so much larger than forces elsewhere, it would have been impossible to supply those sized units elsewhere in the globe. The WWI actions in the African colonies or between Japan and China were effectively inconsequential, so for historical purposes, it is better to leave them out. Having a worldwide variant available for fun is fine.
Many historians (and not just American ones) feel that the entry of the US into World War I was decisive. Germany had knocked Russia out of the war, and while both the German and Anglo-French armies were significantly worn down, the stalemate with the front lines well into France could have led to much more favorable peace resolution than what was achieved at Versailles, had the arrival of the US troops not tipped the balance. The support of the British Empire are implicitly modeled in giving England three supply centers, but the arrival of the yanks is not. Again, this could be a minor variant, modeled somewhat abstractly. If Germany does not employ submarine warfare, then the British Empire supplies more resources (the Scottish supply center) to England, or if German did unleash the U-Boats, then eventually the Americans get angry and England gets two extra units.
The scale of the units in Europe being so much larger than forces elsewhere, it would have been impossible to supply those sized units elsewhere in the globe. The WWI actions in the African colonies or between Japan and China were effectively inconsequential, so for historical purposes, it is better to leave them out. Having a worldwide variant available for fun is fine.
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