Every one knows the 300 spartans were defeated. So what happened after their defeat?

Answer 1

Aftermath
When the body of Leonidas was recovered by the Persians, Xerxes I, in a rage at the loss of so many of his soldiers, ordered that the head be cut off and the body crucified. This was very uncommon for the Persians; they had the habit of treating enemies that fought bravely against them with great honor, as the example of Pytheas captured earlier off Skyros shows. However, Xerxes I was known for his rage, as when he had the Hellespont whipped because it would not obey him.

Xerxes I was curious as to why there was such a small Greek force guarding Thermopylae and interrogated some Arcadian prisoners. The answer was that all the other men were participating in the Olympic Games, a very important event for them. When Xerxes I asked what the prize for the winner was, "an olive-wreath" came the answer. Upon hearing this, Tritantaechmes, a Persian general, said to Mardonius: "Good heavens! Mardonius, what kind of men are these against whom you have brought us to fight? Men who do not compete for money, but for honor".

After the departure and defeat of the Persians, the Greeks collected their dead and buried them on the hill. A stone lion was erected to commemorate Leonidas. Forty years after the battle, Leonidas' body was returned to Sparta where he was buried again with full honors and funeral games were held every year in his memory.

The simultaneous naval Battle of Artemisium was a stalemate, whereupon the Athenian navy retreated. The Persians were now in control of the Aegean Sea and all of peninsular Greece as far south as Attica. The Spartans prepared to defend the Isthmus of Corinth and the Peloponnese, while Xerxes I sacked an evacuated city of Athens, whose inhabitants had already fled to Salamis Island. In September, the Greeks defeated the Persians at the naval Battle of Salamis, which led to the rapid retreat of Xerxes I. The remaining Persian army, left under the charge of Mardonius, was defeated in the Battle of Plataea by a combined Greek army again led by the Spartans, under the regent Pausanias

SOURCE AND ADDITIONAL READING
http://en.wikipedia.org/wiki/Battle_of_Thermopylae

Answer 2

The rest of the Greek army retreated and regrouped. The Spartans gave them time to pull off the tactical fall-back.

Answer 3

Actually, you are not correct. The Spartans won. They achieved their goal, and they saved Greece. You should not consider the sacrifice of their lives to achieve such a great goal as defeat.

Answer 4

After their defeat they came under the rule of Thebes, then under Philip of Macedon. Later they became a part of the roman empire in 146b.c. Alaric 1,Visigothic king destroyed the city a.d. 346 and Slavonic tribes settled in Laconia.