At first I'm ready to say Pearl Harbor but at least the Japanese were identifiable and all in the same location. This new kind of warfare has no identity and is harder to stop because its an ideology.
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Funny you should mention ideology because that is what drove the Japanese to attack in the manner they did. Of course as you also said they were identifiable and terrorists since 911 have all been underground for the most part.
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They are the not the same, 911 was an infiltration job and Pearl Harbor was an outright military attack. Once targeted civilians the other targeted military vessels.
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