"Athena, goddess of wisdom and war, was the guardian of the city [of Athens], and she had offered it the gift of the olive tree. Since it takes many years for olive trees to bear plenty of fruit, the planting of so many olive trees in Athens indicates that people had hope for the future and they had found ways to feed themselves until then.
    "This is also the reason why the olive branch became a symbol for peace. If it takes two decades for your trees to bear a substantial harvest, you are extra vulnerable to warfare that might wipe out all your investments in one moment. Therefore olive growers usually insisted on negotiations and reconciliation when city-states were at each other's throats, and the olive came to symbolise both commerce and peace."

~ Johan Norberg on free trade as a powerful palliative for conflict and war. From his book Peak Human: What We Can Learn From the Rise dnd Fall of Golden Ages [hat tip Tony Morley]