alright I know I said I wouldn't make custom flags but idk man
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nations will have the ability to switch between ideologies (usa/ussr's switch will obviously end the rp though and gb/france will take a stuffload of influence so that'll never happen) but some can become influenced enough that they can reform their government to accommodate the pressure or they can shut everything out and declare themselves nonaligned
shown below are the flags of japan between these three phases

democratic (rising sun with mount fuji in foreground)

communist (olive branches, although reflecting a western influence, can also represent a desire for a prosperous empire)

nonaligned (the imperial seal, used as a callback to the japanese isolationist movement that existed from its founding until the mid-victorian era)