The Greco-Persian Wars were a series of conflicts between the highly decentralised Greek city states (which ranged from proto democracies like Athens to uber Fascist Sparta) on the one side and the Persian Empire on the other side. The Persians were liberal in at least one way, they abolished slavery but the Persians had an autocratic empire and subdued rebellions, see Persian Empire.
Aside from various wars in which the Persians conquered the Greek states of Ionia, there are three wars that are still remembered today.
In the first war the Persians invaded by sea but were defeated by the much smaller Athenian army at the battle of Marathon. An unsuccessful Athenian plea for help from the Spartans is still commemorated and re-enacted as an Olympic sport.
The Battle of ThermopylaeEdit
The second war was a Persian invasion by land with an enormous army supported by a large fleet. The epic battle of the second war was the Battle of Thermopylae, in which 300 highly trained Spartans killed thousands of poorly trained Persians and got killed themselves. The Persians then went on to destroy Athens, but failed to catch the Athenians who crossed over to the island of Euboia, and who destroyed the Persian fleet at the battle of Salamis. The rest of the campaign saw a propaganda battle as the Persians learned that the Spartans they'd fought weren't some sort of elite among the Spartans but merely "fathers of sons"; When the Spartans finally decided to send their whole army out to fight many more Greeks joined them and at the battle of Plataia the Persians broke easily. The legacy of the second Persian invasion includes some of the worlds finest architecture in the Acropolis of the rebuilt Athens, and a triumph of the Athenian commoners who rowed the galleys at Salamis, these rowers were the basis of the Athenian "democracy" in which you didn't need to be rich enough to own fancy armour in order to be a citizen, just male and not a slave (by modern standards quite a Conservative setup - but misogyny and slavery were ubiquitous in most countries in that era, democracy was a brave new venture).
Alexander the GreatEdit
The third memorable Greco-Persian war was a Greek invasion of Persia, in which Alexander of Macedon became known as the Great as he conquered the whole Persian Empire. His conquests included modern day Turkey, Syria, Israel, The Lebanon, Egypt, Iraq, Iran, Afghanistan, Pakistan and large chunks of sundry surrounding states such as Uzbekistan. Despite some notably thuggish behaviour such as the destruction of "fabled ancient Marrakanda" (modern Samarkand), Alexander is usually remembered as an example of a gracious conqueror who celebrated cultural diversity and was happy to respect any local religion that acknowledged him as at least partially divine. His early death precipitated the usual Macedonian tribal conflict that happened if the King died without leaving a strong heir, except this tribal conflict was fought by large armies across a vast Empire. One of his generals, who was perhaps also his illegitimate half brother, Ptolomy didn't try and fight for the whole Empire, and instead modestly settled with becoming a god ruling Egypt as its Pharoah and founding the Ptolomaic dynasty.
Aside from explaining how Kleopatra and her Greeks were still ruling Egypt in Julius Caesar's time, the main legacy of the Greco Persian wars was that Athenian ideas of democracy didn't get totally snuffed out, and the military lesson that if you want a seriously kickass army that can conquer Turkey, Syria, Israel, The Lebanon, Egypt, Iraq, Iran, Afghanistan, Pakistan along with large chunks of sundry surrounding states such as Uzbekistan, get yourself a gay general and lots of gays in your army.