So the Persians landed at Marathon in the fall of 490 BC. When the Athenians heard that the Persians had landed, the citizens (the free men) met in their assembly to decide what to do. A lot of Athenians were very frightened of the Persians, and some thought they should just surrender before the Persians killed them all. Other Athenians thought they should give up having a democracy and just go back to an oligarchy that would be able to run the war better. But when they voted, the Athenian men decided to keep their democracy, and to fight back against the Persians.
So the Athenians got together their army and marched out to meet the Persians. They would have been safe behind the big stone walls of their city, but they chose to march out to battle anyway. Maybe the Athenians were afraid someone would open the door for the Persians the way it had happened in Eretria.