Human beings are not designed to keep secrets. Every aspect of our being is shaped for the sharing of information—through speech, gesture, facial expression, posture, and every other deliberate or inadvertent sign of emotion and intent.
Thus it should not surprise us that every would-be dictator, tyrant, conquered, prophet, colonizer, politician, artist, and dog-catcher in history clearly signaled his intentions long before he acted, and in plenty of time for others to prevent them. Neither Hitler nor Churchill, neither Pol Pot nor Abraham Lincoln, ever did anything they hadn’t told us and shown us they would do.
That they are rarely prevented has more to do with our inattention, cowardice, or ambition to ride his coattails than with his particular skill. Dog might run from the dog-catcher as soon as they see the net, but they rarely tear out his throat and kill him, which is, of course, the only rational course of action for the dog that values its life, liberty, or happiness.