Herodotus, in his The Histories, connects the performance of the Athenians in the battle of 506 BCE to their democracy. Here’s an excerpt from The Histories (Book 5, Chapter 78):
The Athenians at this point became much stronger. So it is clear how worthy an object of attention is equality of public speech not just in one respect but in every sense. Since when they were ruled by tyrants, the Athenians did not stand out from their neighbors in military capability, but after disposing the tyrants, they became overwhelmingly superior.
This, then, shows what while they were oppressed, they were, as men working for a master, cowardly, but when they were freed, each one was eager to achieve for himself.According to Herodotus, people who are not free cannot be expected to do their best in a battle. He thinks that the Athenians won the battle because they were the collective masters of their own collective fate. They were not being ruled by a tyrant.