Why should foreigners feel compelled to take action against President Assad of Syria when he kills 1,000 Syrians with poison gas and not when he kills 100,000 with guns and other “normal” weapons? One good reason is that it is possible for countries to enforce a ban against chemical weapons and it is not possible to enforce a ban against fighting and killing with ordinary weapons.

There are now, and always have been, a number of countries where large numbers of people are being killed by governments or other fighting forces. Generally the people responsible for the killing have strong reasons for what they are doing. It seems essential for their safety or to achieve their goals. Therefore in most cases outsiders need a force that is more powerful than the local forces to stop the fighting.

Only rarely will there be a number of countries with military forces that are willing to send them into combat in a foreign country to stop internal fighting in that other country. And only very rarely will there be non-military measures that are strong enough and can get wide enough support to make fighting forces stop fighting. Therefore the ”international community” often does not have the ability to prevent or stop large numbers of people from being killed by governments or internal conflicts. That is a fact of life in the world as it is, and as it likely to continue to be for some years.

In contrast, the use of poison gas is rarely essential to forces engaged in internal combat. Gas is only slightly or moderately more effective than normal weapons. Therefore, while it may be useful, few potential users will find it essential. That is, commanders with everything at stake for themselves will ask themselves whether the advantages of using chemical weapons are great enough to justify the costs that may be imposed on them for doing so. And often it will be politically feasible for foreign countries to do something to hurt anyone who uses poison gas enough so that he would be better off not using the gas.

In other words, because the military benefits of using chemical weapons are small, the limited actions which foreign countries are prepared to make to stop gas warfare can be powerful enough to deter the use of gas.

Furthermore there is already a substantial degree of international agreement that chemical weapons should not be used. All except seven UN members are party to the Chemical Weapons Convention. And it is supported by widespread feelings of abhorrence – whether or not those feelings are logical.

Therefore it is feasible to have and maintain a largely effective ban against the use of chemical weapons. And it is not now feasible to have an effective ban or policy against large scale killing in internal warfare.

Since there is widespread agreement that the use of chemical weapons is a bad thing, and since it is feasible to largely prevent the use of such weapons, it makes sense to continue international programs against poison gas – even though there are worse things that the international community cannot prevent.

The international program to prevent the use of poison gas may have an additional advantage. If experience shows that a ban – and taboo – against chemical weapons is effective, it may somewhat strengthen similar international efforts against nuclear and biological weapons.

Unfortunately the reason why a ban against nuclear weapons (NW) is more valuable than a ban against chemical weapons is also the reason why it is harder to stop people from using nuclear weapons. Nuclear weapons are normally much more powerful than ordinary weapons. Therefore they can be essential. And if using a NW is essential to someone’s safety or success it is harder to organize measures strong enough to deter the use of NW. But not necessarily impossible. However NW are not the subject of this article.

Biological weapons are also not the subject here. They are different. In some ways more like chemical weapons, in others more like NW.

In brief, it is worthwhile for the US to act in Syria in a way that makes everyone see that Assad ‘s decision to use chemical weapons hurt him much more than it helped him. Afterwards people should ask themselves, “why was he so stupid as to use poison gas? It led to his downfall,” or, at least, greatly increased the danger of his defeat.