Which weapons are illegal?

As I have noted in previous posts, I think that intervening in the Syrian civil war is a bad idea. This is for several reasons but chiefly because no American interest is at stake. No vital economic resource is at stake; no ally is being threatened; there is not even genocide being committed in Syria. As a result of this the Obama administration is reduced to arguing for retaliation to defend the Chemical Weapons Convention of 1925, which Syria is not even a party to. This would be like attacking India or Pakistan or, God forbid!, Israel for violating the Nuclear Non-Proliferation Treaty of 1968 that they are not signatories to.

Let's look at chemical weapons and why they were banned. They were first used in World War I by the Germans in April 1915 on a large scale, although the French might have used chemical grenades before then in the war. The Allies quickly developed their own chemical weapons--blister agents designed to burn the skin and tissues upon contact. Both sides experimented with various blister agents (mustard gas, chlorine, phosgene) and improved the effectiveness of the weapons. But simple gases masks rendered the blister agents much less deadly than conventional artillery. By 1918 chemical shells were almost half of the shells fired at the start of both the German spring offensive in March-April and the Allied offensive in July-August. But despite this chemical weapons accounted for only two percent of total casualties in the war and one percent of total deaths. Conventional artillery accounted for over three-quarters of all battlefield deaths with machine-guns accounted for the majority of the remainder. Britain suffered less than 6,000 combat deaths due to chemical weapons.  This was compared to over 180,000 total British casualties due to chemical weapons. Chemical weapons were mainly a psychological weapon meant to induce fear and a means of denying space to the enemy because of their persistence. It was like sowing a minefield by artillery and particularly useful in taking out the enemy's artillery at the start of an offensive. But because of shifting winds the use of chemical weapons could easily backfire on the army employing them.

After the war thousands of chemical weapons survivors clogged the military hospitals of the combatant nations. Long-term care for these veterans was expensive especially for the European countries with their ruined economies. So it was relatively easy to agree to a convention banning this one class of weapons, even as the real killers (artillery, machine guns, submarines, bombers) continued to be developed and perfected by defense industries in Europe, the United States, and Japan. 

Since World War I chemical weapons have mainly been used by totalitarian dictatorships against their own populations or against civilians in countries they were conquering. Violators of the chemical weapons prohibition "norm" include Nasserist Egypt, Saddam Hussein's Iraq, Vietnam, and the Soviet Union. 

The first conventions banning certain classes of weapons were the Hague Conventions of 1899 and 1907. These banned such quaint classes of weapons as hollow-point bullets designed to explode upon impact. But shrapnel was considered to be perfectly legal. Later on bullets were designed that tumbled when they impacted flesh because they were traveling at a slower rate than they had been traveling in the air. These would cause multiple wounds. But these bullets are perfectly legal. So what is banned is usually something that is psychologically frightening and against the notions of honor of the warrior class at the time, but not terribly useful militarily. It is the equivalent of legalizing prostitution and bordellos but forbidding in them certain sex acts that are unspeakable in polite company. 

So the big question now is if Assad publicly announces that his military is using exploding bullets will we lob a few cruise missiles into Syria as punishment?