What did King Leonidas and the brave 300 do?

Answer 1

They helped 7000 Greek warriors from other cities hold the pass at Thermopylae to hold up the Persian army and force a sea battle in the nearby strait at Artemesion. When the Greeks lost the sea battle, the pas was abandoned, but the Spartan and Thespian contingents continued to hold the pass to let the other Greek soldiers escape before the Persian cavalry broke through and rode them down in open country. They went down to a man, bravely sacrificing themselves to let their comrades get away safely.