Why didn't the Germans fully exploit the first gas attack near Ypres

Question

I'm reading up about the second battle of Ypres (April-May 1915) which started with a chlorine gas attack north of the Ypres-salient.

The attack caused a gap of several miles (3 to 4) wide to be opened. Their only obstacle to capturing Ypres (& overrunning the entire salient) was the G.H.Q line. The G.H.Q - line although ideally situated on top of the Pilckem Ridge wasn't adequately staffed at the time and could thus be taken if attacked by a large enough force.

But I read that the Germans didn't have enough reserves to deploy & to capture Ypres, let alone exploit the breakthrough to its fullest. Nor were they apparently fully aware of the potential effects/success of the gas attack. They wasted precious time observing the effects of the gas before attacking with infantry. In the end the opportunity to move in and capture Ypres was lost.

The gas itself was already used in a combat situation at Bolimow (though it partially failed) and was probably (couldn't find records on this) tested in labs. So they must have known the potential of the weapon.

So this leaves me with the following question: Why didn't the German High Command foresee the possible effects of the gas? And why play out your trump card (the gas) in the first place, if you don't have the manpower to fully exploit a possible breakthrough?

Answer 1

My favorite source for all WW1 information is the YouTube channel The Great War, which follows the events of the war week by week exactly 100 years later.

They have an episode for the week gas was first used in a large scale, the week from the second battle of Ypres, and a special on just poison gas. These will be my sources for my answer.

The first large scale gas attack took place in January of 1915 by the Germans against the Russians, but because it was deployed in the frozen eastern front, it wasn't very successful due to the gas failing to vaporize. This was tear gas, not chlorine, mind you. Chlorine was first used in the second battle of Ypres, as you've been reading. The French saw the advancing yellow cloud, and figuring it was a smoke screen meant to obscure advancing troops, sent their troops to the area to defend. Because it was poison, this is what opened the giant hole in the lines that you mention in your question. The second episode linked goes into this at about 2:30. It was combination of reasons:

• The gas attacks on the Eastern front weren't successful, so they weren't expecting it to have a huge impact.
• It is the first time that Cl gas had been used, so they weren't sure what to expect.
• They didn't expect the French to clump together in front of the cloud, maximizing its effectiveness.
• Since it was just an experiment, they didn't expect such a large hole to be opened in the lines, and didn't know what to do.
• The Germans were lacking in reserves, which had just been transferred to the Eastern front to assist in fighting Russia.
• The Canadian troops nearby fought bravely to defend the exploit, then quickly developed counter measures. They covered their faces with cloths soaked with urine, and had goggles. Although there was still a high casualty rate in the battles that followed, it was enough to allow them to function.