At the time of the battle of Marathon, no Greek state had a standing army. Partly, this was because the state apparatus required to mobilise financial resources for the maintenance of armies had not yet reached the required level of development. Partly, it was because Greek political communities had grown up around the idea that military service and citizen rights were directly connected. Only those who served in defence of the community were allowed to have a say in the way it was run. This concept of citizenship made a standing army unnecessary - the entire adult male population could be mobilised at need - but it also made people suspicious of a system that would limit military power to a small elite. What if these people were to decide they were the only ones who deserved political rights? Who was going to stop them?

In some Greek states, like Sparta, an elite of wealth had limited all political power to its own class - but this was justified by the role these wealthy citizens played in defending their community. Sparta, like the other Greek states, had no army. It just had a large citizen body that lived off the labour of an enslaved population, making their hands free to serve the interests of the state. This relative freedom allowed the Spartan citizen body to spend more time to prepare for war; they were consequently better at it than other Greeks, whose armies consisted of the whole population in arms.

For the battle of Marathon, the Athenians mobilised their entire hoplite levy. According to later sources, they were able to field about 9,000 hoplites of their own. This is plausible, since we hear that they had 8,000 hoplites at the battle of Plataia 11 years later. But this very large number of hoplites (the largest of any Greek state at the time except Sparta) would have included a very broad swathe of the civilian population - all those who could afford hoplite equipment. They ranged from small farmers and skilled workers who could only just afford a shield and spear, to the very richest men in Athens. Their most prominent general was Miltiades, from the stupendously wealthy and influential clan of the Philaidai. Among those in the ranks were the playwright Aischylos and his brother Kynegiros, who was killed in the fighting. However, in later times, common men from the many towns and communities of Attika claimed to have been Marathonomachoi, Marathon-fighters, who had stood up to the Persians in a united defence of their democracy.