The specter of chemical weapons has been hanging over war-torn Syria for months.
Although viewed with skepticism by U.S. officials, the latest claims and counterclaims by the Syrian government and opposition forces over their alleged use in Aleppo province and a Damascus suburb have intensified concerns and prompted the United Nations to promise an investigation.
Syria's government insists it doesn't have chemical weapons, and wouldn't use them against its own people if it did, while the Syrian opposition says it neither has such munitions nor the means to make them.
Whatever the truth of the latest allegations, military analysts say they believe Syria may have one of the largest stockpiles of chemical weapons in the world. Specifically, the supply could include sarin and VX gases -- both nerve agents -- and mustard gas, which are banned under international law.
The prospect that these could potentially be deployed by an increasingly beleaguered regime has made many observers anxious -- and has been cited as a "red line" for robust action by the United States.
So why would Syria have such armaments?
Few munitions evoke as much fear as chemical weapons. And unlike nuclear weapons, they are relatively inexpensive to develop and stockpile.
This lends them a disproportionate importance for Syria and the region, analysts say.
"In the Middle East, chemical weapons have been seen as a possible counter to Israel's nuclear weapons," said Dr. Susan B. Martin, of the Department of War Studies at King's College London.
"They are seen as a possible strategic deterrent," she said, "and they are cheaper and easier to have than nuclear weapons."
Several countries in the Middle East have refused to sign the Chemical Weapons Convention until Israel signs the Nuclear Non-Proliferation Treaty.
Dina Esfandiary, a research associate at the International Institute for Strategic Studies, says Syria in particular embraced a chemical weapons program as a way to bolster its strategic strength despite economic weaknesses, especially after Israel imposed a series of humiliating military defeats on the Arab world.
"The best way to operate asymmetrically was for Syria to have its chemical weapons program," she said.
The Syrians started working on research and development of such munitions in the 1970s and have continued to invest in the program since, said Esfandiary.
It's difficult to quantify how large its stockpiles are, but experts believe that Syria has the largest program in the Middle East and the fourth largest in the world.
"The important thing in terms of Syria is that it requires very little investment, very little technology and is very easy to conceal," said Esfandiary.
"It's like a 'wild card' -- it's the core of Syrian security policy because it prevents Israel doing anything too rash."
How have they been used in the past?
Unfortunately for mankind, chemical weapons are not a new threat.
Close to a century has passed since their devastating deployment by both sides on the battlefields of the First World War prompted widespread revulsion. The United Nations Office for Disarmament Affairs estimates that nearly 100,000 deaths resulted from their use then.
Efforts have been made since 1918 to stamp them out -- through the 1925 Geneva Protocol, which bans their use, and the 1997 Convention on Chemical Weapons, which bans their use, development, production and transfer -- but still some nations stockpile them.
Much of the work in developing and manufacturing chemical weapons occurred during the Cold War period.
The UNODA estimates that by the 1970s and '80s, 25 states were developing chemical weapons capabilities, and that since the end of the First World War, such munitions have caused more than 1 million casualties globally.
Unlike much of the rest of the world, the Middle East has witnessed the use of such weapons in recent decades -- by Egypt against Yemen in 1963, as well as by Iraq against Iran and its own Kurds in the 1980s, Martin said.
Perhaps the most notorious instance is their use by Saddam Hussein's regime against civilians in the Iraqi Kurdish city of Halabja.