As the fifth century B.C. opened, the Persian Empire was moving westward in a quest to consolidate control over the whole of the Near East. Then, in 499 B.C., reacting against the demand for taxes and tribute, the Greek city of Miletus rose up against Persian rule. This revolt spread from Miletus to the other parts of Ionia. Athens sent a naval force from across the Aegean Sea and led a small force that took the Persian provincial capital of Sardis in the same year.
In order to gain vengeance against Athens, the Persians, who lacked a substantial navy, would themselves have to cross the Aegean Sea. In 491 B.C., the Persian emperor Darius sent envoys to Athens, demanding its submission to Persian rule. The Athenians not only rejected Darius's offer but also killed the envoys. The Persian army landed at Marathon in 490 B.C., where the Athenians waited five days to engage the Persian forces for fear of being outflanked by the Persian force. There are different theories as to why the Persians themselves did not engage, including the idea that the marshy land on the plain took away one of the Persians' major advantages: their cavalry. In any case, being a great distance from home and having limited supplies, the Persians could not play the waiting game forever. Some historians believe that the Persian commanders decided to send the cavalry to Athens by sea to take the undefended city. This move may have precipitated and, indeed, necessitated the Athenian attack. The ensuing Athenian victory was as swift as it was unexpected. According to this narrative, the Athenian forces then marched back and were in a position to defend their city by the time the Persians with their cavalry completed their sea voyage.
Although geography certainly played a role in some of the main battles, so too did the tactics and strategy of the Greeks; therefore, geography cannot be seen in isolation. Darius's son Xerxes gathered a force of unprecedented size for a new invasion of the Greek mainland in 480 B.C. Rather than crossing the Aegean, the Persians approached from the north, where they had domains in Thrace and Macedonia. At the highly consequential Battle of Salamis, which prevented the invasion of the Peloponnese, the Persians were baited into entering the Straits of Salamis, where their large numbers served as a disadvantage: they became disoriented and fell prey to Greek attacks. At the Battle of Plataea, the Greeks resisted the temptation to attack the Persians on good cavalry terrain and instead, by retreating, took the fight to the hills and were victorious. So geography often helped in combination with Greek military strategizing and decision-making.