<P> This allowed diversification of the allied armed forces, rather than simply mustering a very large hoplite army . The visionary Athenian politician Themistocles had successfully persuaded his fellow citizens to build a huge fleet in 483 / 82 BC to combat the Persian threat (and thus to effectively abandon their hoplite army, since there were not men enough for both). Amongst the allies therefore, Athens was able to form the core of a navy, whilst other cities, including of course Sparta, provided the army . This alliance thus removed the constraints on the type of armed forces that the Greeks could use . The use of such a large navy was also a novelty to the Greeks . </P> <P> The second Persian invasion is famous for the battles of Thermopylae and Salamis . As the massive Persian army moved south through Greece, the allies sent a small holding force (c. 10,000) men under the Spartan king Leonidas, to block the pass of Thermopylae whilst the main allied army could be assembled . The allied navy extended this blockade at sea, blocking the nearby straits of Artemisium, to prevent the huge Persian navy landing troops in Leonidas's rear . Famously, Leonidas's men held the much larger Persian army at the pass (where their numbers were less of an advantage) for three days, the hoplites again proving their superiority . </P> <P> Only when a Persian force managed to outflank them by means of a mountain track was the allied army overcome; but by then Leonidas had dismissed the majority of the troops, remaining with a rearguard of 300 Spartans (and perhaps 2000 other troops), in the process making one of history's great last stands . The Greek navy, despite their lack of experience, also proved their worth holding back the Persian fleet whilst the army still held the pass . </P> <P> Thermopylae provided the Greeks with time to arrange their defences, and they dug in across the Isthmus of Corinth, an impregnable position; although an evacuated Athens was thereby sacrificed to the advancing Persians . In order to outflank the isthmus, Xerxes needed to use this fleet, and in turn therefore needed to defeat the Greek fleet; similarly, the Greeks needed to neutralise the Persian fleet to ensure their safety . To this end, the Greeks were able to lure the Persian fleet into the straits of Salamis; and, in a battleground where Persian numbers again counted for nothing, they won a decisive victory, justifying Themistocles' decision to build the Athenian fleet . Demoralised, Xerxes returned to Asia Minor with much of his army, leaving his general Mardonius to campaign in Greece the following year (479 BC). </P>

What did greek city states rely on for protection