Battlefield commanders will only use them when all else has failed and when, with disaster looming, they want to avoid defeat. Such weapons have little other purpose other than to terrorise those on the receiving end - and even then they can be counter-productive by producing adverse propaganda. That helps to explain why the United Nations is seeking "clarity" over what happened when missiles with chemical warheads were used in the Ghouta area near the Syrian capital of Damascus.
With no side claiming responsibility and with various claims and counter-claims, the blame can only be divided between government and rebel forces. Both have their reasons to use chemical weapons and equally both have reasons not to use them.
The Syrian Army
They remain the principal suspects, as the regime has been supplied with chemical weapons by Russia and has already made threats to use them. They also possess the relevant technology and the delivery systems, and in the increasingly bitter civil war they have shown that they are unafraid of causing high civilian casualties in high density centres of population, such as Aleppo and Homs. Double bluff could also be a factor, with the chemical weapons being used while the blame is switched to the rebel forces. Or president Bashar al-Assad could have simply ordered their use to prove that the international community was toothless and would not respond - just as Saddam Hussein did in Iraq in the 1990s.
In the area of eastern Syria that was being contested last week, government forces were in the ascendancy and they had no good tactical reason to use such uncompromising weapons in a built-up area containing large numbers of civilians. The presence in Damascus of a United Nations inspection team could also represent a good reason for not taking the risk of using chemical weapons. Propaganda might also have played a part: at a time when the Assad regime has been vilified in the international community, the last thing it needs is further evidence of civilian suffering and pictures of dead women and children.
The rebels' failure in Aleppo earlier this year proved that while their forces are able to take ground they are unable to hold it. Using chemical weapons in the Ghouta sector of Damascus would allow them to take ground at little cost to themselves. The use of a missile delivery system would also give notice that they are acquiring modern weaponry and are able to meet the government forces on increasingly equal terms. If the weapons are found to have come from government arsenals, it could suggest divisions within the Assad camp.
Although the jihadist elements in the Free Syrian Army are known to be ruthless fighters who will stop at nothing in their quest for victory, the use of chemical weapons would be regarded by many supporters as a step too far. The evidence of suffering civilians, especially women and children, would make nonsense of their claims to be fighting on behalf of a better and fairer Syria. It is also difficult from a purely military point of view to understand what would be achieved by using chemical weapons in a seemingly arbitrary way. Weapons of this kind are generally deployed in overwhelming force against specific targets where large numbers of opposition forces can be eliminated, not in crowded urban areas which are subject to varying weather conditions.