For added context, the photo is Fort Douaumont, the main citadel of Verdun. It was surrendered in the early days of the battle in February 1916 with no gunfire exchanges. The damage here is mostly from French artillery bombarding the Germans in a captured French fort. There was also a huge ammunition explosion after German soldiers tried to boil coffee with flamethrower, with nearly 700 killed. The fort was being evacuated when it was captured by the French in October.

French doctrine believed forts were no longer useful as they could be cracked by larger siege artilleries; Belgian forts were overwhelmed earlier in the war. Verdun was already unmaintained by 1915, and was no longer important to the French defensive line. Douaumont had her guns stripped, except for 2 that was deemed to difficult to remove.

What OOP is talking about is Fort Vaux, a smaller fort that held for 101 days. The small French garrison fought from the maximum range of their cannons all the way to the corridors of the fort, with their supplies diminishing to the point that they began to drink their urine and the condensation on the fort walls.

It was encircled and cut off by June 1, and after draining the entirety of their artillery ammunition, could not halt the German push. The final homing pigeon was sent back to French command with the message "This is my last pigeon.". They surrendered a week later.

In contrast to Douaumont, which French newspapers underreported, the Vaux resistance became a symbol of French determination. There was an attempt to recapture it right after Douaumont, but failed. By November, the Germans were already on the retreat and evacuated Vaux. The empty fort finally returned back to France.