Final answer:
Xerxes, the king of Persia in 480 BCE, defeated the Greeks at the Battle of Thermopylae.
Step-by-step explanation:
The person who defeated the Greeks at Thermopylae was Xerxes. In 480 BCE, Xerxes I, who was the son and successor of Darius I, launched a massive invasion into Greece. The Spartans, under the command of King Leonidas, made a valiant stand at the narrow pass of Thermopylae to delay the Persian forces. Although the Spartans were eventually surrounded and defeated due to a betrayal, their sacrifice was instrumental in galvanizing Greek resistance and securing time for Greek preparations, which eventually led to the Persian navy's defeat at Salamis.