479 BCE. The Persians had taken over Athens and were determined to do the same to all of Greece - spurring The Battle of Plataea (led by Mardonius, who was put in charge by Xerxes), which followed the Greek naval victory at Salamis
The Greek obviously didn't want this, so they set up camp on high ground to guard Plataea from the Persian army.
For eight days, neither army did anything.
Eventually, the Persians sent a cavalry unit to attack the Greek but were beat, prompting the Greek to move to a better spot.
In response, the Persians intercepted the Greeks' food supplies and blocked them off from their only water source, Gargaphia Spring.
The Greeks were forced to retreat, but the were disorganized and separated. One group ran into Mardonius's cavalry unit and forced to fight against the Persians' significant numbers advantage and archers.