Why did the battle of Marathon happen

In the late 6th Century BCE, Persia took Asia Minor into its empire. It included the Greek cities dotted around its coast and islands. These cities had been colonised by the cities of mainland Greece, and when they tried to revolt against the Persians they appealed to their mother cities for help. In 499 BCE the cities revolted and the following year Athens and Eretria sent an army to support Miletus. They overdid it by capturing and burning the Persian provincial capital of Sardis, and were singled out by Persian King Darius I for retribution as an example to others to stop interfering in the peace of his empire.

In 490 BCE Darius sent a punitive expedition against them. Eretria was betrayed and the inhabitants sold into slavery. The force then landed in Athenian territory on the plain of Marathon. Its plan was to capture Athens and install as Persian puppet the ex- tyrant of Athens Hippias who had earlier been expelled from the city.

The Persian infantry were to stay and keep the Athenian army interested while the cavalry was sent around by sea to gallop up to the city through the gates opened by Hippias’ supporters. The Athenian army would then be trapped between the city and the infantry.

The Athenian infantry, lurking in the hills behind the Marathon plain for fear of the Persian cavalry, saw it leaving and ran down to defeat the Persian infantry. Then realising what the cavalry was up to, ran back over the hills to their city and formed up in front of it just as the cavalry disembarked ready to gallop up through the gates. The disappointed Persians, having lost much of their army, went home. Darius decided that the only way to stop the mainland Greeks from medlling in his empire was to take it all over, install puppets and so establish an ethnic frontier. But that is another story about the 480 BCE invasion.