The game can end prematurely if Napoleon is forced into exile. This occurs if Paris is captured and not recaptured by your forces in a certain number of turns.
If you defeat Great Britain and subdue, if necessary, each of the remaining European powers then France is considered to be at peace with all Major Powers. I think that you cannot be at war with the Ottomans, either.
You can subdue a hostile power by defeating it, of course. On the other hand, if you are at peace with, say, Austria, then you'll stay that way unless you declare war or the AI succeeds in bringing that power to belligerent status.
If Great Britain is conquered it makes it much easier for you because the AI can't change any powers to belligerency against France.
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