Draw a line in the sand

To draw a line in the sand means to set a boundary or limit, beyond which, one will not go and is first attested from the 1950s. There have been some etymological attempts to ascribe a literal meaning to lines drawn in the sand by swords in ancient times. Perhaps Spartans at Thermopylae, for example, did draw a line in the sand with their swords signifying no retreat. There is also the story of a Roman general drawing a line in the sand with his sword and daring an enemy to cross it and so start a war against Rome. More recently, Colonel William Travis, the commander of the Texan forces at The Alamo in 1836, is said to have drawn a line in the sand with his sword, signifying the intent to fight to the death against the besieging Mexicans. There is no convincing evidence, however, that the current figurative use, as an idiom, dates from such ancient events. That is why the OED, among others, opts for a transitional, idiomatic origin from c. 1950.