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I don't think anyone has thought of this yet, but I'm just tossing it out here:

When Catherine the Great arrived in Russia, she fell ill to a bout of pleurisy which almost killed her. However, she survived and went on to marry the future Tsar of Russia Peter III, who was soon afterwards deposed, mainly due to his pro-Prussian views, leaving Catherine to become Empress of Russia.

My idea is: what if her bout of pleurisy had killed her, preventing her from becoming Empress or even marrying Peter III? How would this have affected his reign? Would the bloodless coup launched against him still have succeeded, and if so, who would have taken the throne? If the coup failed or did not go ahead, how would Peter the Third's reign have turned out? Would he continue to support Prussia as an ally rather than an enemy? Would he have gone ahead with his proposed invasion of Denmark, and if so, could he have won? Would he have been able to transform Russia from a feudal state to an advanced and democratic nation?
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