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[Eagle of Fire]

Well, I can't talk about Imperialism but in Imperialism 2 it make sense that all the big powers suddenly jump on you since the whole goal of the game is to be the winner among those big powers. All game long, all you do is build strength on the new world to be able to do just that.

But if you don't want other powers to jump on your it is easy: just either bribe or trade them out from attacking you.