Final answer:
Among the Greek city-states, only a small force from Plataea joined the 10,000 Athenian hoplites to fight the Persians at the Battle of Marathon in 490 BCE.
Step-by-step explanation:
In the year 490 BCE, when the Persians invaded and the pivotal Battle of Marathon ensued, Athens stood almost alone. While the Athenian forces fervently prepared to defend their city-state and democratic ideals against a formidable Persian force, the Athenians sent Pheidippides to Sparta to request aid. However, the Spartans were celebrating a religious festival and were unable to send help until several days later. Consequently, by the time of the battle, only the Athenians (about 10,000 hoplites) and a small force from Plataea are recorded to have actively engaged the Persians at Marathon.