The Obama administration is calling for Congress to approve the use of military force in Syria because the Syrian government allegedly used chemical weapons against civilians in August, killing about 1,400 and injuring scores more.

So what are chemical weapons, and why are they so terrible? The Organization for the Prohibition of Chemical Weapons defines them as “any toxic chemical or its precursor that can cause death, injury, temporary incapacitation or sensory irritation through its chemical action.”

The 1925 Geneva Protocol was the first international law to prohibit the use of poisonous gas as a weapon of war. The 1993 Chemical Weapons Convention took it a step further and asked signatories to stop producing chemical weapons and to safely destroy existing stockpiles of chemical arsenals.

Both the United States and Russia, the two countries with the largest stockpiles of chemical weapons, signed the convention and are still in the process of destroying their arsenals -- a slow, careful containment process that must be overseen by the OPCW.

Syria, North Korea, Egypt and Angola are the only countries that have not yet signed this treaty. Israel and Myanmar (Burma) have signed the treaty but have not ratified it.

Here is a map of all the countries that have had chemical weapons in the past, might currently have them, or definitely do have them. Click on any country for more info: