What was the purpose of using Poison gas?

Answer 1

Tactically, the use of gas weapons is to make territory unavailable to the enemy, typically for a short period of time.

In this role, chemical weapons are of limited effectiveness and debatable tactical worth. There are a few reasons for this:

• Chemical weapons that disperse into the atmosphere are highly dependent on prevailing wind conditions and, if the wind shifts, can roll back over your own forces.
• Persistence of airborne chemical weapons is variable, depending on weather, which makes planning difficult.
• Chemical weapons that adhere to the ground surface can be extremely hard to clean up, thus denying the area to invading troops (and everyone else) until extensive NBC contamination can be conducted.
• Most chemical weapons have military countermeasures. This means that chemical weapons are far more effective against civilian population than against prepared military units.
• Chemical weapons are more effective against stationery or slow-moving forces. In today's modality of warfare and since WW-II, this is the exception and not the rule.

Even in WW-I, which was a splendid example of static ground emplacements, checmical weapons were not very effective. Both sides had effective countermeasures, and varible weather conditions made these weapons a 1-edged sword. Also, phosgene, the most effective gas used in WW-I, accounting for 80% of the deaths attributed to gas, had roughly a 24 hour onset time, which means that it didn't do much at first, and this complicated planning.