The French Army's reinforcement to Colonel Delion was not as easy as imagined. The German Army had surrounded the fortress and occupied the surrounding high grounds, making it impossible for reinforcements to approach without being intercepted.
Soon after, another distressing occurrence was reported: the Germans had mixed gas shells with their artillery shells. People were being poisoned without realizing it, and the symptoms were unlike anything seen before. This indicated that the Germans had invented a new type of poisonous gas.
(Above is a phosgene shell, used by both sides during World War I, with an estimated usage of 100,000 tons.)
More terrifyingly, the gas was being delivered via artillery shells rather than gas canisters as before.
The German Army was clearly prepared.
Whether the Germans were using gas shells or not, the French Army needed to wear gas masks because no one knew if the shells fired at them contained gas.
