Interpretation
The demand for earth and water symbolized that those surrendering to Persians gave up all their rights over their land and every product of the land. Giving earth and water they recognized the Persian authority over everything; even their lives belonged to the king of Persians. Then negotiations would take place to specify the obligations and the benefits of the liegemen.
The phrase earth and water, even in modern Greek, symbolizes unconditional subordination to a conqueror.
According to the modern historian J. M. Balcer, the significance of earth and water is that they were Zoroastrian symbols and representative of vassalage to the Persian Empire. "Persian heralds traveled throughout Greece demanding the recognition of Persian Suzerainty and the Zoroastrian symbols of earth and water, the marks of vassalage...".
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