Simplify the following text for a Grade 12 native English-speaking person.
Original: The Athenian general Themistocles had proposed that the allied Greeks block the advance of the Persian army at the pass of Thermopylae , and simultaneously block the Persian navy at the Straits of Artemisium .
Simplified: The Athenian general Themistocles suggested the Greek allies should block the Persian army at the pass of Thermopylae , and block the Persian navy at the Straits of Artemisium .