What happened is Xerxes was afraid there are more Spartans waiting around for an ambush, and they were just as strong as the 300. Eventually he did march in, and Xerxes burned down Sparta and Athens and all the rest. The battle at the end I believe was the Battle of Marathon, where the story of Nike came from. It wasn't the full force of the Xerxes army, and I'm quite sure it wasn't just the Spartan army either. The Greeks barely had shit for armies because they had been in a state of civil war for many years, and eventually banded together for a common cause. Xerxes was eventually repelled not from land forces but in a navy force where his fleet went against an Athenian fleet. It was rather like the 300 spartans, because they basically channeled the Persian army into a very small area of water, and had rams attached to their smaller, more manueverable ships. They ended up battering the shit out of the Persian fleet, taking out something like 1/4th of the fleet. Xerxes retreated.

And there were no fucking Vikings. Wikipedia it if you want the true historical account. Though it doesn't seem like it, the movie is actually a bit historically accurate, but there are quite a few gaps in it. It would've been a much longer movie and not called 300 at all if it focused on the rest of Greece. It was a movie centered upon the Spartans at the Battle of Thermopylae and the inspiration to the Greeks, NOT Xerxes and his invasion.

Anyway, I may have a few facts mixed up. Wikipedia it if you're really interested, and yes, wikipedia really is a reliable source in the matter.