Answer:
When his father died Xerxes I decided to continue the war with the Greeks. He spent 10 years preparing his army and navy to return to Greece. The Greeks, however, were prepared for the Persians to return. They united with the Spartans who were known for their military skill. After the Battle of Plataea, the Persians lost and were forced to leave Greece.
(please read explanation for reasoning)
Explanation:
(This paragraph is talking about the Second Persian invasion of Greece)
When his father died __Xerxes I__ (Cyrus The Great, Darius I, Xerxes I, and Cambyses) , decided to continue the war with the Greeks. He spent 10 years preparing his army and navy to return to Greece. The Greeks, however, were prepared for the Persians to return. They united with the ___Spartans___ (Spartons, Athenians, and the Babylonians) , who were known for their military skill. After the Battle of ___Plataea___ (Marathon, Plataea, and Thermopylae) , the Persians lost and were forced to leave Greece.
First blank: Darius I died, son, Xerxes planned for second invasion
Second: specifically the Spartans, although both the Spartans and Athenians were important allies to Greece
Third: the Persians won Thermopylae and although Athenians did win the battle of marathon, that was during the first Persian invasion. Xerxes, as also described in the text given [without a name], was returning to Greek--and his father died in the first Persian invasion. (I know that this war is specifically the second invasion because of the first sentence in the paragraph.)
(If you want to know more, the Wikipedia page for the Second Persian War describes the event far better than I do here)
