Leonidas was a king of the Spartans during the mid-1st millennium BC.
In 480 BC, Leonidas led a force of Spartans in defense of a mountain pass against a vast Persian army during the conflict later known as the Battle of Thermopylae. According to Julian Bashir, for two days the Spartans, under Leonidas, led a "heroic struggle" until they were wiped out, "fighting to the last man."
In 2375, Bashir offered Ezri Dax the chance to simulate Leonidas' fall at the Battle of Thermopylae, after his long-time holodeck companion Miles O'Brien had left Deep Space 9, comparing the scenario to the struggle found at Battle of the Alamo. (DS9: "What You Leave Behind")