The cartoon shows French President François Hollande, who is due to visit Mali today, drawing a line in the desert sand with a stick. A French armoured vehicle is parked behind him. A truckload of al-Qaeda jihadists has just made a U-turn, and is heading off in the opposite direction, angrily brandishing their weapons.
The cartoon is a visual representation of an English idiom. If you draw a line in the sand, you establish a limit beyond which things will be unacceptable.
President Francois Hollande drew a line in the sand Friday against al-Qaida-linked militants in Mali who have been advancing toward its capital: France will be ready to intervene to stop any further advance.
—The Seattle Times, 11 January 2013
According to Wikipedia, the origin of the phrase is unknown, but the Oxford English Dictionary suggests a transitional use from 1950, and a figurative use only as late as 1978: