That's not a line in the sand. That's a range marker! 

A line in the sand. 

The enemy host approached 
Beheld the line in the sand 
Walked along it without crossing 
Deciding what to do next 

Dug into chilly foxholes 
Defenders awaited onslaught 
Their sights set to the distance 
At which the enemy paused 

As bugles signaled advance 
A volley ripped into the ranks 
And while the bodies were falling 
Lead storm tore at them once again 

Checked by effective fire 
Invaders retreated in shame 
Learning that line in the sand 
Was also marker for range 

Remember that, would-be tyrants 
To boundaries pay some respect 
For free men still have their rifles 
And wages of evil is death