Thucydides understood the paradox of how defense could be considered aggressive more than 2400 years before the first nuclear explosion over Hiroshima.

Most Greek city-states did not have impregnable walls protecting them. Mutual vulnerability was already functioning as a deterrent to substantial warfare. No city-state could attack another without suffering painful retaliation. Each city-state, even the strongest, was exposed to the “second strike” capability of its opponents. A “first strike” would automatically trigger a response with “unacceptable damage” for the attacker, thus leading him to think twice before launching an attack.

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