Sparta and Athens were natural allies who provided support to each other - Sparta leading the Dorian bloc of independent city-states, Athens the Ionian bloc, with both interested in maintaining peace.
So when a Persian expedition was sent to subdue Athens in 490 BCE after its interference in the Ionian Revolt against Persia, Athens naturally asked Sparta for support (however Athens had defeated the Persians at Marathon by the time the Spartans arrived). When Persia sent a full scale invasion in 480 BCE, the southern Greek city-states united to repel it under the leadership of Sparta with Athens providing the largest naval component.
They remained friends and allies until 460 BCE, when Athens sent an army to help the Spartans put down a revolt of its serf population. During this campaign they fell out and avoided each other. After the Persians agreed to peace the Athenians turned the Delian League into an empire of its own, became over-adventurous and clashed openly with the Peloponnesian League led by Sparta, resulting in the 27-year Peloponnesian War which Athens lost.
Self defence.
They were already allies, Sparta had sent its army to aid the Athenians ward off the Persian attack in 490 BCE at Marathon (it arrived too late for the battle), and along with the rest of the southern Greek cities, they had a vested interest in uniting to repel the Persian invasion of 480 BCE which was aimed at bringing the Greek cities under local tyrants responsive to Persia.
Pony express
The Persians who was led by Xerxes .
Exercising his power of king through his generals.
daa Sparta brought the soldiers ndd the Athens brought theirr navy (:
Persian
Threat of annihilation by the Persian invasion
He was an orator who was persuasive enough to garner popular support - eg getting the Athenians to spend the proceeds of a rich silver strike, which they intended to divide up amongst the populace, on building a strong navy which helped turn back the Persian invasion.He was an astute commander, planning the response of the Greek cities to the Persian invasion and persuading the other cities to follow his plans. As part of this he persuaded the Athenians to abandon their indefensible city, send their families to shelter in other cities, and take to the sea to fight there in the ships he had provided.He persuaded the Athenians to clearly establish their independence - facilitating the rebuilding and extension of the city walls destroyed by the Persians, which gave Athens independence from influence by Sparta.His popular support was based largely on his populist approach - preferring to influence affairs by appealing to the people rather than trying to persuade the aristocracy. When the aristocrats regained control as a result of their leadership during the Persian war, they exiled him - the went to Argos, was implicated in a plot and fled to Asia Minor where Persian king Artaxerxes gave him governorship of a province.
He built the Royal Roads
Against the Persian invasion.
It established 20 provinces each under a Persian governors who were responsible to the king and his council.