A family member suggested this expression, and the explanation of its meaning and origin on Wikipedia (under line in the sand) is interesting. The first meaning is a point “…beyond which one will proceed no further.” The second is it is a point at which a decision is permanent and irreversible. There is a Biblical link in John 8:6 where “…Jesus draws a line in the sand to address those who are about to stone a woman caught in adultery. There are several references to “…historic, or legendary, military events…”  (including the Alamo) in which a version of the expression was said to have been used.