Other copy at Legion of Honor, Fine Arts Museums of San Francisco, CA.
It is about Maréchal Pétain’s supposed order to Joffre (see Wikipedia: Although Pétain did say "On les aura!" (an echoing of Joan of Arc, roughly: "We'll get them!"), the other famous quotation often attributed to him – "Ils ne passeront pas!" ("They shall not pass"!) – was actually uttered by Robert Nivelle who succeeded him in command of the Second Army at Verdun in May 1916.)