Welcome to CFC SaMx.
I have a few thoughts about "the most realistic WW2 scenario ever"....
Because in Civ3 you are only allowed to place aircraft in your own cities/airfieds, how are you going to represent the mightly power of the American 8th Airforce if the United States and Britain are different civs?
Will the Americans 'own' certain cities in England?
Or will the British be allowed to build American aircraft and station them in their own British cities?
A possible solution would be to combine the Americans, British and Canadians into one big "Western Allies" civ.
After all, EVERY operation in Europe, EVERY sea bourne landing, EVERY campaign, was a joint Anglo-American one.
Then you could have these "Western Allies" only able to build British units in 1939, and later, when America enters the war, have this civ able to build both British and American named units.
Also, because England is relatively small, they could have a limited production base....meaning that most of the big production cities will be based across the Atlantic, and most units will be built there and have to be shipped by sea to Britain. (representing 'Lend lease', the Battle of the Atlantic, and give the German U-boats something to torpedo.
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If you remove the 'allows sea trade' from Harbours, then you could arrange it so that American units can only be built in America, Canadian units are only built in Canada, and British units are only built in England.
(To reduce corruption, the 'Forbidden Palace' would be in Washington.
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These "Western Allies" could have two different types of heavy bomber:-
*British Lancasters (weak defence, but give them the 'stealth' ability....changed from 5% to say 25%....to represent the difficulty of stopping night bombers)
*American B-17 Flying Fortresses (better defence, but also the 'precision bombing' ability, to reflect their more accurate daylight bombing effects)
So....what should the "Western Allies" build....Lancasters, who are hard to stop but you can't depend upon them hitting a specific target, or B-17's, who will suffer heavy casulties (at least until 'Escorted Bombers' come along), but can be used to hit specific buildings.
(Decisions, decisions......)
As for leader-heads....
If you arrange it so that the scenario starts in say the Industrial era in 1939, then Winston Churchill will be visible.
But once the "Western Allies" gain enough techs to be able to enter the next era, then the era popup picture could be "AMERICA ENTERS THE WAR!", and now President Roosevelt is shown as their leader (representing how the Americans became the dominant partner in the alliance).
All this would also stop Britian and America from forming a 'seperate peace' with Hitler, as well as stopping them from declaring war on each other!
(With no way to set diplomacy in Civ3, who knows what the AI will do!
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Just a few thoughts.